It's more a trademark issue, which requires continued use (but that may still be legally satisfied even with a low active user base). If there is an active maintainer/owner, they probably could file a trademark suit.

Copyright in anything like code or content will outlive us all in Berne Convention countries. That protection is automatic, so the notice like you see there is not necessary, but it also indicates dates that begin the copyright clock, not the ending of it.

As someone that used to use xchat regularly, I feel very old seeing just how long it has not even been updated...

also obligitory fuck elon and his stupid rebrands