The historical nature of or convention on the web has been first come first served on names. Most well designed systems will never change a username due to inactivity or for any reason other than the owner requesting it (even that’s not allowed on most systems). Usernames that aren’t actively used, after a known period of inactivity, may be deleted along with content associated with that username.

Look at email addresses: platforms like Gmail and Yahoo will delete your account for inactivity with adequate advance warning, but they will not release your email address for someone else to claim in the future (on the other hand, paid platforms like Fastmail and Posteo will allow your deleted addresses on their domains to be grabbed by anyone).

With GitLab’s policy of renaming inactive accounts without any warning or notice, any links that someone has shared could later point to a different person. A “404 Not Found” error or an “email address does not exist” error is far better, IMO, than redirecting someone to a totally different person.

If an inactive username is to be released back for use by someone else, there should be a significant duration from the time the username is disabled to the time when it’s available for someone else.