Yes I see the issue here now, we are still showing the Discourse Connect button even if the invitation has already been redeemed, and also for an existing user who has not redeemed the invite.
Invite not yet redeemed for existing user with DiscourseConnect enabled:
The UI change sound pretty straight forward, I know how annoying this is, but if it is just hiding a button conditionally I recommend doing it, but not backporting.
So sorry, I think @hyphalos can simply use CSS here to work around, no? Hide the elements that are a problem? I would prefer not to backport anything we are too close to 3.0 release.
This is not really doable since a different button needs to be hidden depending on state:
However if its too risky before 3.0 then there is not much we can do here, I will just make the fix in main and it will trickle down to stable in the new year.