Some of the madhabs such as the Hanafis and Shafi'is consider these to be recommended optional prayers. They come under the category of nafal or ghair mu’akkadah type of sunnah.
The evidence for prayer before Asr includes:
رحم الله امرءا صلى أربعا قبل العصر
May Allah have mercy upon a man who prays four before Al-Asr
— Jami at-Tirmidhi
And for before Isha includes:
روي عن عائشة رضي الله عنها أنه عليه السلام كان يصلي قبل العشاء أربعا
Aisha narrated that the Messenger of Allah ﷺ offered four rak’ahs before Isha
— quoted in مراقي الفلاح شرح نور الإيضاح
They are also evidenced by the general meaning of the hadith:
بين كل أذانين صلاة ... لمن شاء
There is a prayer between every two calls (i.e. Adhan and Iqama) ... for the one who wants to pray.