I think that if we get flags to tag each completed feature from the last 13 years this forum has been active because people have OCD, it would be a big waste of time for everyone.
The tag itself is newish, never widely adopted, and we could simply close topics for completed features, allowing everyone to create new topics and quote old ones as necessary.
I’d rather remove the completed flag. It was applied 400 times and was only created as a way to help people write changelogs, who are dealt with differently nowadays.