As someone who builds the banking apps of joinatmos.com, uses linux on desktop and aspires to use it on phone - I’ve been thinking about this too. Our react native build won’t support Linux (say, Flathub) very well at all, and building a separate GTK app is not cost effective for a startup. Flutter could work, or if we’d made our mobile app a wrapped website in Cordova or whatever that would have been an option I believe too.

For now, our mobile Linux users would be “stuck” with the mobile web experience, though we tried our hardest to make it a performant PWA that you can pin to your home screen. The main thing you lose is persistent biometric login, though many privacy conscious users seem to not trust biometric unlocking of your session in the first place (they mistakenly think their Face is being uploaded to our servers, which is of course not how the technology works on iOS/Android) so that may not be an issue.

Edit: From my time trying to use a dumbphone as a daily driver a few years ago, cash apps were a bigger problem (Venmo, etc) as theres a social component to “not paying someone back until later because I dont have a smartphone”.