You can sort of tell when someone is gone because of “last post” (visible on user card) and “seen” (visible on user page) stats.
If someone has not been seen for quite some time, you could mark their account either on the user card, or in their user name next to all their posts, but that’d be super noisy.
I’ve also seen some sites graph this as a sparkline next to each user showing their .. I dunno, post rates over time or attendance rates. If the sparkline flatlines.. you know they are not there without needing to be told that.