How to enable or disable XDG autostart functionality on KDE Plasma Desktop/mobile?

On systemd, we want to use systemd unit files for user services when those are provided by upstream/packaging. We already have a way to do that on GNOME, this issue is about making sure we know how to do it on Plasma too.

On GNOME, you can tell the DE to skip starting a service via XDG autostart by setting the following key/value in the XDG autostart .desktop file: X-GNOME-HiddenUnderSystemd=true.

@PureTryOut asked a KDE developer about doing this on Plasma and was told that we should use X-systemd-skip=true. According to the systemd docs, this key/value is used to configure systemd-xdg-autostart-generator. This generator will read in xdg autostart .desktop files and auto-generate a systemd unit file for it.

I (@craftyguy) found this response to be a little confusing, so need to look into it more. The generator is orthogonal to what we're trying to do with controlling the DE's xdg autostarter mechanism. Does Plasma re-use this key/value internally in some other way than what the systemd docs say? What mechanism in Plasma even does the XDG autostarting of apps, and can it be configured?

I tried to ask this in the Plasma matrix channel, sounds like a good next step is reading KDE systemd startup blog post and asking further questions in the kde-devel matrix channel1).

Also, I didn't actually try setting X-systemd-skip=true on a plasma install w/ systemd and poking around... Maybe it works? If so, how?