Just noticed this change on rawhide...
* systemd-logind will now by default terminate user processes that are
part of the user session scope unit (session-XX.scope) when the user
logs out. This behavior is controlled by the KillUserProcesses=
setting in logind.conf, and the previous default of "no" is now
changed to "yes". This means that user sessions will be properly
cleaned up after, but additional steps are necessary to allow
intentionally long-running processes to survive logout.
While the user is logged in at least once, user@.service is running,
and any service that should survive the end of any individual login
session can be started at a user service or scope using systemd-run.
systemd-run(1) man page has been extended with an example which shows
how to run screen in a scope unit underneath user@.service. The same
command works for tmux.
After the user logs out of all sessions, user@.service will be
terminated too, by default, unless the user has "lingering" enabled.
To effectively allow users to run long-term tasks even if they are
logged out, lingering must be enabled for them. See loginctl(1) for
details. The default polkit policy was modified to allow users to
set lingering for themselves without authentication.
Previous defaults can be restored at compile time by the
--without-kill-user-processes option to "configure".
I did not see any discussion about this particular setting in the
systemd-devel mailing list