I don't know about Android but Apple users can literally start adopting TOTP without changing a single thing.
Providers should simply add instructions telling people that if they have an Apple device they can just go to the keychain and add the code displayed on the screen or use the QR with the camera
"Feature phone" can often have some android installed on it. From all definitions I ever saw, it was mostly about form factor (keeping that old Nokia looks with physical buttons). Nokias now have something called KaiOS, unix-based OS where you can develop just like elsewhere.
Porting some app into another OS would be probably a showstopper due to budgets/deadlines, even though even my old Nokia in 2006 could easily run java apps like these (but ended up mostly running Snake et al).
I don't know, which people were laughed at in the industry for the last few decades because they wanted to have control over their own computers and run their own code on their own machines?
I don't know about Android but Apple users can literally start adopting TOTP without changing a single thing.
Providers should simply add instructions telling people that if they have an Apple device they can just go to the keychain and add the code displayed on the screen or use the QR with the camera