Systemd can be configured to kill all processes belonging to a user when he or she closes the last login shell. Some people had this enabled on their servers and were surprised that their tmux/screen sessions kept disappearing.

This is a bit disingenuous; people didn't just "have it enabled", it was made a default despite a lot of concern that it was such a radical departure from the normal behavior.

Some people? Do you mean everyone who's distro did not change the default value?