When a user has been deactivated, they can no longer be @mentioned in a post (the pop-up does not help with auto-completion). This makes sense to a large degree, as that user will not be notified. (That said, it could be argued that it’s still be nice to be able to reference them easily via a mention and perhaps find some other way to indicate they are no longer active.)
But in the case of search I have found myself often wishing I could more easily find posts from users who have been deactivated.
Search does properly filter to posts by a deactivated user if you type the user name perfectly, but it’s much more difficult to do that without auto-complete.
I think it’d make sense to have search assist with autocompletion here by default (a refinement would be that it only works if the user has made any posts).