How to run systemd user service to trigger on sleep (aka. suspend, hibernate)?

sleep.target is specific to system services. The reason is, sleep.target is not a magic target that automatically gets activated when going to sleep. It's just a regular target that puts the system to sleep – so the 'user' instances of course won't have an equivalent. (And unfortunately the 'user' instances currently have no way to depend on systemwide services.)

(That, and there's the whole "hardcoding $DISPLAY" business. Every time you hardcode session parameters in an OS that's based on the heavily multi-user/multi-seat Unix, root kills a kitten.)

So there are two good ways to do this (I suggest the 2nd one):

Method 1

Create a system service (or a systemd-sleep(8) hook) that makes systemd-logind broadcast the "lock all sessions" signal when the system goes to sleep:

ExecStart=/usr/bin/loginctl lock-sessions

Then, within your X11 session (i.e. from ~/.xinitrc), run something that reacts to the signal:

systemd-lock-handler slock &

xss-lock --ignore-sleep slock &

(GNOME, Cinnamon, KDE, Enlightenment already support this natively.)

Method 2

Within your X11 session, run something that directly watches for the system going to sleep, e.g. by hooking into systemd-logind's "inhibitors".

The aforementioned xss-lock actually does exactly that, even without the explicit "lock all" signal, so it is enough to have it running:

xss-lock slock &

It will run slock as soon as it sees systemd-logind preparing to suspend the computer.