same question. My solution is: find someone who has installed the old version of testflight, log in to his/her account to download the old version of testflight, and then switch back to my own account.

But this should not be borne by users, and not everyone can find such a person. Even if Apple is unwilling to provide newer versions of the system to some old phones, at least in this case, Apple should allow everyone to download applications that conform to their system version.

I am doing related testing works for iphone6plus. There are still many people using this phone in 2021. I hope Apple will not ignore these users.