I'm using the FUTO keyboard and disable the AOSP keyboard in Settings->System->Keyboard->Onscreen-keyboard, but each time I change User the AOSP keyboard gets re-enabled.
I guess this might have been a temporary fix to the issue of not being able to unlock the phone (if using a password and having an inoperable 3rd party keyboard).
If that's the case, it'd be nice to be able to properly disable the AOSP keyboard, at least for non-Owner users, because there are other workarounds now.