I remember reading on HN at the time, how the systemD folks opened a bug report (or some such similar) on the tmux github because systemD was disappearing those

I remember reading on HN at the time, how the systemD folks opened a bug report (or some such similar) on the tmux github because systemD was disappearing those tmux sessions, and their solution was for tmux to change how they managed persistency. Tmux stood their ground, and I think that's why KillUserProcesses=yes isn't still default. (iirc)