I conjecture that the bank didn’t bother to test the app on a new version of iOS until this week, found a major bug, and knew they couldn’t fix it fast enough.
But they could have tested three months ago when the beta came out, and any major issues (like using an old API) probably would have occurred then.
If I coax a random FOSS Mac app project from the early to mid 2000s into compiling on modern macOS there’s usually deprecation warnings left and right, with many of the deprecations having happened in OS X 10.2-10.6 (well over a decade ago), but it’ll still work fine.
So devs usually have plenty of chance to move things over. The grace period just isn’t indefinite like it is with e.g. Windows — sometimes the deprecated APIs are removed 5-6 releases down the road and so it’s a good idea to move off of them sooner than later.
I am asking to learn.