Is SimpliSafe compatible with MAC 10.15.XX

SS3 has no interaction with a computer at all, so the question is meaningless.

SS2 has a USB resident "setup wizard" which historically has had serious problems with Mac.  Since they never fixed it and the system is no longer being updated, it would be safe to say that it probably won't work with recent Mac updates, and doesn't work well even ones which were out in the SS2 heyday.

Well the web interface works fine on a Mac, yes.

P.S. It actually would be nice if there were a native Mac app. I get the impression that the iOS app connects directly to the base station over the LAN (?), but the web app requires a round trip (or more) to SimpliSafe.com, which is both slower and less secure.

My iPhone with the latest IOS 13.6 works fine with the SimpliSafe app.