RetroWar is offically supported on Windows 10, but if you have a Mac you may have noticed Steam will download and play a Mac build. This is provided on a best effort basis. (It may not be up to date, and if Apple decides to break it in 6 months it could stop working.)
As of today there is also a Linux build! It should be considered a beta, which is why it is not advertised as supported in the store. But if we get enough feedback from Linux users we would like to eventually promote it to supported status.
RetroWar on Linux and Mac
Source: Steam Community (opens in a new tab)