I believe that I still have Opera Mini on my 3rd Generation iPad which is also stuck on iOS 9.3.5. But if I ever delete it, it would be gone and there would be no way to reinstall it without jailbreaking the device (and even that might not work).
The key with old iOS devices is therefore to keep old apps around as long as you can, because they can't be reinstalled past a certain point.
Obviously there is some mild security risk involved in the fact that many of these apps are no longer being updated...but then the whole OS is not receiving security Apple updates either because Apple chooses not to provide any. It should still be fine for casual use (just don't put any sensitive information on it).