The verse you've quoted yourself is the most explicit reference to "prisoners of war" that I am aware of.
وما ملكت يمينك مما أفاء الله عليك
and those your right hand possesses from what Allah has returned to you [of captives]
— Quran 33:50 ; Word for Word
The verb أفاء is derived from the root فيء , and is used as a synonym for war booty (غنيمة) as should be verifiable from any dictionary:
Lane's Arabic-English Lexicon on فيء:
Lisān al-ʿArab by Ibn Manzur from archive.org digitised text from shamela.ws:
والفيء : الغنيمة والخراج . تقول منه : أفاء الله على المسلمين مال الكفار يفيء إفاءة
[ فيء is غنيمة and خراج (kharaj), Allah returns the wealth and possessions of the Kuffar to the Muslims ]
وقوله مما أفاء الله عليك أي رده عليك من الكفار . والغنيمة قد تسمى فيئا ، أي مما أفاء الله عليك من النساء بالمأخوذ على وجه القهر والغلبة .
The Saying: مما أفاء الله عليك meaning the women of the disbelievers who are returned to you, as spoils (غنيمة) are sometimes called في meaning that the women who Allah has given to you as spoils of war captured through conquest.
— Tafsir Qurtubi
Examples of this usage within the Quran to refer to spoils of war, note that a concordance of أفاء is found here and a concordance of all derivatives of the root is found here:
وما أفاء الله على رسوله منهم فما أوجفتم عليه من خيل ولا ركاب ولكن الله يسلط رسله على من يشاء والله على كل شيء قدير
And what Allah restored [of property] to His Messenger from them - you did not spur for it [in an expedition] any horses or camels, but Allah gives His messengers power over whom He wills, and Allah is over all things competent.
ما أفاء الله على رسوله من أهل القرى فلله وللرسول ولذي القربى واليتامى والمساكين وابن السبيل كي لا يكون دولة بين الأغنياء منكم وما آتاكم الرسول فخذوه وما نهاكم عنه فانتهوا واتقوا الله إن الله شديد العقاب
And what Allah restored to His Messenger from the people of the towns - it is for Allah and for the Messenger and for [his] near relatives and orphans and the [stranded] traveler - so that it will not be a perpetual distribution among the rich from among you. And whatever the Messenger has given you - take; and what he has forbidden you - refrain from. And fear Allah ; indeed, Allah is severe in penalty.
— Quran 59:6-7 ; Word for Word
والمحصنات من النساء إلا ما ملكت أيمانكم
And [also prohibited to you are all] married women except those your right hands possess.
— Quran 4:24 ; Word for Word
Which makes sense for freshly enslaved prisoners of war from the Dar-ul-Harb, as otherwise it would be Zina.
Further verses where the context of ملكت أيمانكم (right hand possesses) is clearly slaves:
And those who seek a contract [for eventual emancipation] from among whom your right hands possess - then make a contract with them if you know there is within them goodness and give them from the wealth of Allah which He has given you. And do not compel your slave girls to prostitution, if they desire chastity, to seek [thereby] the temporary interests of worldly life. And if someone should compel them, then indeed, Allah is [to them], after their compulsion, Forgiving and Merciful.
— Quran 24:33 ; Word for Word
The preceding verse to the above mentions عِبَادِكُمْ and إِمَائِكُمْ and this verse mentions فَتَيَاتِكُمْ , all of which are nouns for "slaves", so the context dictates that مَلَكَتْ أَيْمَانُكُمْ also means slaves. It also mentions the possibility of compelling the فَتَيَاتِكُمْ and earning from them, so it alludes to slaves as a free person can not be legally compelled or earned from.
The next verse is:
And whoever among you cannot [find] the means to marry free, believing women, then [he may marry] from those whom your right hands possess of believing slave girls ... But once they are sheltered in marriage, if they should commit adultery, then for them is half the punishment for free [unmarried] women.
—Quran 4:25 ; Word for Word
This verse mentions "right hand possessions" as distinct from محصنات (muhsinat) and having half the legal punishment (of adultery) compared to them.
فعليهن نصف ما على المحصنات
Then for them is half the punishment of the free woman
Muhsan\Ahsan, literally complete\perfect\excellent, has multiple meanings depending on the context: It means chaste, married, and free (as opposed to enslaved), all of the above or a subset of them.
Reading the verse carefully should make it clear that muhsinat in the above phrase means free, because the context excludes chastity:
محصنات غير مسافحات ولا متخذات أخدان
chaste, not those who commit immorality, nor those who take secret lovers
فإذا أحصن فإن أتين بفاحشة
then when married, if they commit adultery
And what is left is freedom.
Also, the verse again mentions فَتَيَاتِكُمْ (slave girls) as those who your "right hand possess".
The following verses allude to that people feel social and financial superiority to those possessed by their right hand, which doesn't inherently hold true for someone bound by contract or oath:
He presents to you an example from yourselves. Do you have among those whom your right hands possess any partners in what We have provided for you so that you are equal therein [and] would fear them as your fear of one another [within a partnership]? Thus do We detail the verses for a people who use reason.
— Quran 30:28 ; Word for Word
And Allah has favored some of you over others in provision. But those who were favored would not hand over their provision to those whom their right hands possess so they would be equal to them therein. Then is it the favor of Allah they reject.
— Quran 16:71 ; Word for Word