Processes using daemon(3) have been around for 21 years, and are used to certain behaviour. Some of those processes behave badly.
systemd is adding functionality to kill all of a user's process when the session ends, no matter what. The problem is that they don't have any way of telling which processes are behaving badly and which ones aren't, so they're telling everyone (e.g. tmux, screen, etc) that they have to implement changes or they'll be killed too.
Fundamentally, the problem isn't whether the systemd behaviour is right, the problem is that it's a huge breaking change and they're asking everyone else to work around the shortcomings of their approach.
> shell managers like screen and tmux had hacks to make them survive sessions ends in the past
tmux, screen, etc. didn't have hacks to survive sessions; they just did, automatically, because of how things worked. systemd is asking the to add hacks to make them survive session ends from now on, when running specifically on Linux under systemd 230+, because systemd is going to change default behaviour for other people's processes.