Startplasma is the session leader managed by the display manager.
Anything started by systemd

On logout or shutdown we correctly stop the graphical-session target and
then startplasma quits.

If SDDM is restarted or startplasma explodes the order happens in
reverse. startpasma quits, but we also need our units to close.
Otherwise we end up in a situation where we're at the login manager but
with all our background services continuing to run.

Given startplasma starts the plasma-workspace target it's on to make
sure this target gets stopped. This starts a small service that exits if
startplasma quits, the plasma-workspace serivce is bound to this and
will teardown if this exits.

This seems a convoluted approach, but it's based on the approach that
Gnome uses called gnome-session-monitor.