They actually added the functionality over 5 years ago. Which is when the tmux project was approached to accept a patch to support pam (not systemd) in order to not be killed.
http://tmux-users.narkive.com/LXp72CHV/pam-support-in-tmux
All they did recently was try to change the default configuration to enable the feature by default.
> 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
Systemd developer asks libc to add systemd specific code
This whole comment thread is about an optional feature that has existed for 5 years and only recently had its default value changed. You are not affected unless you are running a bleeding edge distribution that has already upgraded to a version of systemd released 5 days ago.
I don't think a more long term solution will present itself in the near future and most likely what will happen is that the default will get changed back to disabled.