Yes. Just because it is 50-years-old doesn’t make it suddenly a valid approach. There are 4 states here - process running with usual semantics for closing, process running that would like to linger in the background, and the frozen version of this two. How should a service manager/resource handle — that is very much in-scope for systemd differentiate between a frozen process that should very much be cleaned up in the no-linger case, and any process wanting to linger? And there are well-established ways to determine whether a process is frozen — pinging it expecting a specific answer.

Systemd differentiates between no-linger/linger by introducing user services - one should only write a trivial alias/wrapper script for tmux and can continue to use it to their liking. Also, it has a compile time flag, as well as a runtime config one - it is up to the distro or the user to revert back to the old way.