When I wrote about how applications autostart on modern Linux desktops, there was a Reddit discussion, and one of the people there noted that things could also be autostarted through systemd user units. As covered in the Arch Wiki page on this, Linuxes that are systemd based generally automatically start a 'systemd --user' user systemd instance for you, and one of the things this will do is it will start things in ~/.local/share/systemd/user and ~/.config/systemd/user, which you can manipulate.

However, there are some significant differences between the two that help explain why Linux desktops don't use systemd user units. The big one is that systemd user units are per-user, not per-session. By their nature, desktop applications are a per session thing and so not a great fit for a per-user system. In fact even getting systemd user units to be able to talk to your desktop session takes what is basically a hack, as covered in the Arch wiki section on DISPLAY and XAUTHORITY, and this hack must be carefully timed so that it works correctly (it has to happen before units that need to talk to your desktop are started, and that means they have to be terminated when you log out).

Desktops also have a lot more fine control over what gets started with their current mechanisms. Obviously these things only get started for desktop sessions, not things like SSH logins, and they can be specific to certain desktops or not start in some desktops. I don't believe there is a native systemd unit option for 'run only if this environment variable is defined', so you can't readily make a systemd unit that only runs in desktop sessions, never mind only a particular sort of desktop.

(Relying to any significant degree on user units would also more strongly tie desktops to systemd, although I don't know if that's something they worry about these days or if it's full steam ahead on systemd in general.)

My general impression is that systemd user .service units are not a good fit for what most users want and do with autostarting things today, whether or not they're using a desktop. Systemd user units are probably a better fit for socket and dbus units, because those are more naturally activated on the fly as needed, but I don't know if people are doing this very much (especially for desktop related things).

(As a practical matter, I'd consider it pretty obnoxious if a program decided to set itself to autostart as a systemd user unit. I suspect I'm not alone in this.)