Summing up tests "activating" and "deactivating" @[Mentioned people only] on #mastodon. Apparently there have been some changes.
(from v3 to v4?)
You can't add more people into already existing posts. They won't be able to display it nore get notifications.
(if that ever existed)
You can deactivate/delete mentions, the profile won't get notifications of updates but still can see or search those toots.
note:
still awaiting confirmation of behaviour on #friendica