Believe it or not, "apps" are an important "feature" of a smartphone, even if it's not theoretically bundled with it. Moreover it's not just banking apps, those are just the first ones to go, but any that don't keep backend compatibility will eventually break.
Isn't it the banks/apps that are choosing not to support the phone, not Apple?
It is neither Apple's nor the third party app's obligation to make the third party app work on ancient phones.