The average banking app doesn't communicate via google-operated pipes. Basically the only thing I've seen baking apps do via Google (and I've reverse-engineered quite a few) is the SafetyNet check to ensure their bullshit policy about modified devices. Well, that and notifications, but microG already emulates that library and uses Google's servers afaik.

If a phone manufacturer started shipping Android phones without GApps and they got popular enough, developers would start being more careful with relying on Google Services. We saw this with Huawei - their phones are reall popular where I live and after tons of complaints from huawei users, a bunch of companies fixed their apps to gracefully handle the lack of Google Play Services.