Until recently, network profile URLs were frightening. Five months ago I used one to refer to an active user of Mathematics.
Since a few weeks the URLs are much shorter. Nice! But the old link now redirects me to what I assume is an unrelated user. (Little activity, and no account on Mathematics.)
I think I linked to the author of a question who no longer is a registered user†, and is now displayed as "user6163". That might explain some re-use of network IDs to keep them short. Still then, if doable: the old GUID URL should probably yield a 404 instead?
(Google Cache still knows my old GUID URL, and that works fine. The only other GUID URL I ran into redirects fine too.)
† Given a Meta post about lost reputation for the old user "Nir", maybe some abuse was taken care of, and the current user is some merged account. But then I'd expect some activity in Mathematics again?