KDE Connect: Enabling communication between all your devices

Can this be used without KDE, on another desktop environment? Without systemd, even?

It definitely works on other DEs. As an extreme case, there's even a GNOME extension implementation: https://extensions.gnome.org/extension/1319/gsconnect/

I can't specifically speak to systemd, but it'd surprise me if there was a dependency there; it's not running that deep in the system. Also there's a guix package and they don't use systemd, so that's another mark in favor.

Works even on Windows. So YES