pg_shadow, pg_user and pg_group were added when role support was added, specifically for backwards compatibility. I don't believe there was ever discussion about keeping them because filtering pg_roles based on rolcanlogin was too onerous. That said, we already decided recently that we wanted to keep them updated to match the actual attributes available (note that the replication role attribute modified those views) and I think that makes sense on the removal side as well as the new-attribute side.
I continue to feel that we really should officially deprecate those views as I don't think they're actually all that useful any more and maintaining them ends up bringing up all these questions, discussion, and ends up being largely busy-work if no one really uses them.
Deprecation would certainly seem like an appropriate path for 'usecatupd' (and the mentioned views). Though, is there a 'formal' deprecation policy/process that the community follows? I certainly understand and support giving client/dependent tools the time and opportunity to update accordingly. Therefore, I think having them read from 'rolsuper' for the time being would be ok, provided that they are actually removed at the next possible opportunity. Otherwise, I'd probably lean towards just removing them now and getting it over with.