In a recent update to the HSBC app they’ve added a screen to prevent you from using the app unless you use the default (google) keyboard.
They do a similar thing if you have an accessibility service running that can access the screens content. A fair enough security warning if you’ve happened to install a dodgy keyboard app, but highly frustrating when using an open source alternative that enhances the security and privacy over the default option (HeliBoard in my case).
I haven’t found a way to circumvent the page yet. It would be useful if Android allowed you to block the permission to query all packages, but alas.
It’s possible. First example I can think of is NYT’s games app uses their own keyboard. It’s clunky, but if someone is concerned (or data hungry) enough for the users security they certainly could.