BTW, all the GrapheneOS, etc. are still Android phones.
Obviously this is going to impact the supply of apps, since the market share of custom Android is smaller than even the market share of people willing to sideload or use an alternative store on a mainstream Android phone. Many developers might quit the game.
It does require the developer to make minor adjustments, and most banks are simply too risk averse to agree to doing that (I would know, used to be a senior android app dev at a bank).
[0]: https://grapheneos.social/@GrapheneOS/115062761036828110