Thanks @Conan_Kudo.
Unfortunately, Slackers hate systemd, so it’s seems no ready way to incorporate systemd there.
All I found is systemd packages in Dlackware - Dropline Gnome Slackware, which seems just swaps Slacware’s system init to systemd, for Gnome3 usage. But it is not what I want.
So, my current intention is to run systemd in Docker container in Slackware. Maybe I would have better luck with it.
But to be honest, Snaps dependency on systemd seems awkward for me…
Thanks anyway