As in all former posts there's no clear cut and there seem to be a mixing between the terms fatwa, shari'a and fiqh I'd like to start by explaining the first:
(as for a distinction between fiqh and shari'a you may refer to this meta post)
What is a fatwa? And when a verdict is binding crucial?
A fatwa in Arabic language means giving guidance and showing the true path it also means answering questions that were hard or unsolvable by others.
A fatwa in the terminology of fiqh scholars is answering questions by deducing the relevant ruling of shari'a on a issue, or showing the apparent verdict according to shari'a and it could also be made without a prior question due to an existing or emerging case that needs to be verified.
This last statement shows that fatwas are responses to actually emerging issues and problems like how to deal with qur'an recitation from mp3 or Adhan or a congregational prayer on TV, travelling by plane and the ruling of travel etc..
Here some definitions of scholars:
In the following I'm translating from Arabic language if not claimed otherwise these translations are of my own take them carefully!
قال القَرافي: "الفتوى إخبارٌ عن حكم الله تعالى في إلزام أو إباحة".
Al-Qurafi said: Fatwa is giving information about Allahs ruling (on an issue) be it an order or a permission.
وقال الجرجاني: "الإفتاء: بيان حكم المسألة".
And al-Jurjani said: Giving fatwa means clarifying the ruling (verdict) of a case.
وعرفها ابن حمدان الحراني الحنبلي بقوله: "تبيين الحكم الشرعي عن دليل لمن سأل عنه".
The hanbali scholar ibn Hamdan al-Harani defined it as: clarfiying the shari'a ruling based on evidence(s) for those who asked about it.
From this we may deduce that fatwas are based on ijtihad they may be more correct (close to the truth) and they may be less correct. Therefore the statement of the hadith applies:
"When a judge utilizes his skill of judgement and comes to a right decision, he will have a double reward, but when he uses his judgement and commits a mistake, he will have a single reward."
(In Sahih al-Bukhari and Muslim and elsewhere. Source Riyadh as-Saleheen)
And they are answers of questions and show the verdict. They are not binding.
A fatwā (/ˈfætwɑː/; Arabic: فتوى; plural fatāwā فتاوى) is a nonbinding legal opinion on a point of Islamic law (sharia) given by a qualified jurist in response to a question posed by a private individual, judge or government. A jurist issuing fatwas is called a mufti and the act of issuing fatwas is called iftāʾ. Fatwas have played an important role throughout Islamic history, taking on new forms in the modern era. (Source Wikipedia)
From this we may come to the conclusion that there's no necessity to declare a fatwa obsolete as it is simply not binding! Nevertheless doing fatwa is considered as very dangerous as a person who gives fatwa is a person of trust for all the people who ask him. While Allah the almighty says:
Say, "My Lord has only forbidden immoralities - what is apparent of them and what is concealed - and sin, and oppression without right, and that you associate with Allah that for which He has not sent down authority, and that you say about Allah that which you do not know." (7:33)
And do not say about what your tongues assert of untruth, "This is lawful and this is unlawful," to invent falsehood about Allah . Indeed, those who invent falsehood about Allah will not succeed. (16:116)
and in the sunnah:
Verily, Allah does not take away knowledge by snatching it from the people but He takes away knowledge by taking away the scholars, so that when He leaves no learned person, people turn to the ignorant as their leaders; then they are asked to deliver religious verdicts and they deliver them without knowledge, they go astray, and lead others astray. (Sahih al-Bukhari, Sahih Muslim and elsewhere)
Therefore many great scholars when giving fatwa have felt a big fear or got in a state of fear and many of them were known to answer: "I don't know".
وكان مالك بن أنس يقولُ: (من أجاب في مسألةٍ، فينبغي من قبل أن يجيبَ فيها أن يعرضَ نفسَهُ على الجنةِ أو النارِ، وكيف يكونُ خلاصُه في الآخرةِ، ثم يجيبُ فيها)
Malik ibn Anas used to say: "Anyone who answers a (fatwa) question, he must first check to see whether he is ready for either Hell or Paradise, and find out where his position will be in the hereafter, (only) then he may give an answer."
(Source: Alukah with some edition of mine)
And a good Muslim should be aware that, if he tries to trick the mufti "by beautiful words" which are not as it seems he is only sinning and will be judged upon this:
The Prophet (ﷺ) said, "I am only a human being, and you people have disputes. May be some one amongst you can present his case in a more eloquent and convincing manner than the other, and I give my judgment in his favor according to what I hear. Beware! If ever I give (by error) somebody something of his brother's right then he should not take it as I have only, given him a piece of Fire."
(Sahih al-Bukhari, other versions are also available)
So if the person asking for a fatwa doesn't tell the truth or full information the fatwa could basically be declared obsolete right away.
On the other hand it is essential for a qadi (judge) to be even more careful before speaking out a verdict and it is not wrong if he corrected his verdict due to new information. Therefore 'Umar ibn al-Khattab () when appointing abu Musa al-Ash'ary () as a qadi reminded him in a letter saying (Source as-Sunan al-Kubra of imam al-Bayhaqi):
" أَمَّا بَعْدُ لا يَمْنَعْكَ قَضَاءٌ قَضَيْتَهُ بِالأَمْسِ رَاجَعْتَ الْحَقَّ ، فَإِنَّ الْحَقَّ قَدِيمٌ ، لا يُبْطِلُ الْحَقَّ شَيْءٌ ، وَمُرَاجَعَةُ الْحَقِّ خَيْرٌ مِنَ التَّمَادِي فِي الْبَاطِلِ "
Don't let a verdict you have made yesterday and you came to a result that is closer to truth, as is (but revert or correct it), because the truth is old, and nothing cannot be invalidated by anything. And reviewing and correcting (a verdict) to truth is better than staying (or keeping) falsehood.
(Taken from my former answer on Is there fatwa inerrancy like Papal inerrancy?)
In many schools of fiqh there might be a main line (al-Mo'tamad, for further information see Levels of obligatoriness of rulings within schools of law) when it comes to fatwa on known cases and issues, but nevertheless it is very common to quote other views of single scholars of that school of jurisprudence. These basically are quoted as respected views and sometimes discussed and weighted in front of the accessible sources of information in relevant fiqh books.
Also fatwa means finding solutions or workarounds in a legal manner in some cases as a solution might be an answer of a question see also How to interpret Sufyan a-Thawri's statement on permission in fatwas?.
But I think it is important to say already at this stage that there is an agreed upon fiqh ruling that says there's no ijtihad while there's an existing verdict (order or prohibition) in either the qur'an or sunnah which is agreed upon by all scholars.
لا اجتهاد مع النص
So none can't make zina legal when it is prohibited in the qur'an or order people to pray more or less than five prayers a day when it is clearly defined in the sunnah that there are exactly five obligatory prayers a day or allow riba etc.
While by now we for certain know that a car or automobile is not the creature from earth described in the qur'an and sunnah against any otherwise saying fatwa (See for example Is there any description of the Creature from earth which will appear at the end times?).
Can verdicts vary with time or location?
The simple answer is yes they can and among the first who said so you may find ibn Qayyim al-Jawziyah the student of ibn Taymiyyah. An example of that appears in my answer of What to do after converting to Islam while happily married to a non-Muslim husband? which shows that 'Umar ibn al-Khattab () this already made a difference between women leaving in Bilad al-Kufr and those who emigrated to Muslim land.
But this doesn't mean that giving a modern fatwa show that shari'a has been changed! In fact even the naskh in the qur'an is not an evidence for that. The shari'a of Allah is one ever since the revelation started and it has goals which it pursues by rules, some of these goals were met before the prophet () died (therefore the rulings have been abrogated) others may not have been achieved at the time or even by now, like the elimination of slavery and peace on earth.
In his al Muwafaqat الموافقات imam a-Shatibi said about the perfection of the shari'a:
"Therefore, you can not find therein abrogation after its perfection, specification for its generality, no limitation to its absolute and no cancel for its rulings; not in regard to the legally competent people, not according to special status of some of them, and not according to time without another or condition without the other. What has been confirmed as a cause will not change and likewise for the condition, the obligatory, the advisable and all rulings. They will not vanish or change even if the ordinances are endless." (Source: Alukah)
Changing or adaption of a fatwa does not mean to change the rulings of shari'a, as it can't turn some of them from lawful to unlawful and from an order into a prohibition or the other way around. The meaning -as accepted by scholars- is to change the legal ruling according to the new or different conditions of Muslims whether in time (modern use of materials, or some materials may no more be accessible), location/space (non-Muslim country vs. Muslim country) or people (Muslim minorities, special circumstances of co-existence with non-Muslims).
In fact ibn al-Qayyim when saying that verdicts may change with time and location (space) made a few conditions and limitations for these changes saying (in his Ighathat al-Lahfan إغاثة اللهفان) by classifying rulings into two distinct categories:
"Rulings are of two kinds: A type that does not change not according to time and space or the Ijtihad (juristic effort to infer expert legal rulings) of the scholars; such as the obligations, prohibitions and the prescribed penalties that are ordained by the Shari'ah on crimes and likewise. This kind is unchangeable and no Ijtihad (juristic effort to infer expert legal rulings) can change its type.
The second type is: What can be changed according to the need, time, space and conditions such as the amount of Ta'zir (discretionary punishment), their kinds and descriptions where the Shari'ah changes according to benefit." (Source: Alukah)
So the benefit plays a crucial role in allowing the change of a ruling (fatwa or even juridical verdict). But note still the ruling must go ahead to shari'a so shari'a doesn't change and can't be changed!
In his book "Al Qawa'id Al Fiqhiyyah 'Ala Al Madhhab Al Hanafy wa Al Shafi'y القواعد الفقهية على المذهب الحنفي والشافعي" (The jurisprudence rulings according the hanafi and shafi'i madhhab) Dr. Muhammad az-Zuhayly does a clearer distinction of what ibn Qayyim addressed in a more general context:
Dr Muhammad Al Zuhayly mentioned some disciplines for those rulings that won't change because of time and people. He said:
- The basic rulings which are stated in the Qur'an and the Sunnah that are confirmed by the original texts of Shari'ah: the commands and prohibitions; such as the prohibition of oppression, adultery, usury, wine drinking and robbery. Another example of the basic rulings are the obligation of consent when concluding a contract and the obligation of fighting crimes and defending the rights. Such rulings can not be changed according to time because they are basics ordained by the Shari'ah appropriate to time and generations but only their means change.
- The pillars of Islam and the established facts by Shari'ah can not be changed or altered. They will remain still as were first ordained in the first generation because they can not be altered or changed.
- All the rulings related to worship are not subject to opinion or Ijtihad. They are unchangeable due to time, space, places and people.
- The matters of creed are also firm and steady and neither subject to change nor Ijtihad. They are steady since they were revealed since the time of the previous prophets and messengers until the Last Hour is established. They are not subject to the change of time." (Source: Alukah)
A solution for the linked question
In this Arabic article from 1427 a.H. this means already 13 years ago. The author discusses a proposal three years earlier made by sheikh az-Zindani which he called zawaj friends for young Muslims living in non-Muslim countries this means young boys and girls may get married in a legal manner fulfilling all the necessary conditions of a legal nikah except with living together in an own home -which basically is not necessary, but many schools of fiqh say as along as the woman is in her fathers house he is the prior guardian-. So at the "end of the day" the boy or girl goes back to the fathers house which means that and they have a legal relationship instead of being boyfriend and girlfriend. IMO this is hardly practical as it leaves the bad taste of an upcoming divorce and many Muslim boys come for studiyng to non-Muslim countries and have issues marrying Muslim girls there as their parents prefer marrying them with people from their own culture.