I'm not sure why you're getting downvoted when it's the stated position of the systemd developers that they default to breaking those things. They've acknowledged it publicly. Hell, there was even a thread on the exact issue with tmux 4 years ago: https://news.ycombinator.com/item?id=11797075

Top comment on that link:

    """Salient comment: "Or somebody could go find the actual problem @keszybz saw 
       here - systemd/systemd#3005 - which is: In particular, for my gnome session, 
       if I log out, without KillUserProcesses=yes I get some processes which are 
       obviously mistakes. Even if I log in again, I'm much better starting those 
       again cleanly. fix that, and stop trying to make systemd break the world 
       because somebody's gnome session doesn't currently exit cleanly."""

Or Nicholas Marriott's 9 year old as-yet unanswered questions to the systemd developers, that I copy from a post by hn user JdeBP:

    > "Shouldn't this code be part of glibc instead of tmux?"
    >   -- Nicholas Marriott, 2011
    > If you want to change how processes daemonize, why don't you change how 
    > daemon() is implemented [...] ?
    >   -- Nicholas Marriott, 2016
    > * https://news.ycombinator.com/item?id=11798173
    > * https://news.ycombinator.com/item?id=11798328 

https://news.ycombinator.com/item?id=11798515

You shouldn't be downvoted, this is correct. Sane distros will change the default.

> Sane distros will change the default.

And sane developers don't break multi-decade behaviour in the first place.