See subject. The reason "Switch User" doesn't work is because Cinnamon
implements this feature by running the following command line if the
XDG_SEAT_PATH environment variable is set:

    dm-tool switch-to-greeter

Of course that variable is set because Devuan uses elogind. Cinnamon
does not check for the existence of "dm-tool", nor does it report or log the
failure to run it in any way. When you click "Switch User" on a default
Devuan install, it simply appears to do nothing.

The "cinnamon" package should depend on LightDM.

I have no idea what "dm-tool switch-to-greeter" would do if LightDM was
installed but SLIM was the DM that was running, nor do I plan to test
it.