Earlier quoted context omitted.
nohup is POSIX, systemd is decidedly not. That makes it systemd’s responsibility not to break nohup, and more generally not to require non-admin users to be aware of it.
Nohup is a disgusting hack from a previous century. I don’t see how having less information on what is happening to a process good — nohup interprets supposedly meaningful signals never meant for this misuse differently. If you want to continue to run a process you have to start a service which is exactly that.
SIGHUP is not a signal to shut down, it's a signal that says the controlling terminal is gone. The action taken by the process at that point is the decision of the process and the user that launched it, not the init system. This is exactly what it is for.
Changing that without understanding what people used it for across UNIX vendors for decades, was dumb. And it showed off just how little the writers of the service knew about how users actually use their systems. Huge red flag.
It's not just random long-running batch jobs it killed either, it seems that it would kill all sorts of useful stuff people run for reliability. The irony of the init system killing screen/tmux sessions which people run specifically to keep their shell going in case of a disconnect!