I did my research and it I find it even better than I first thought.
Is Spectrum chat owned IP of GitHub now?
Actually, it is Github who opened and made Spectrum free software by first acquiring the company and then making it fully free and open-source (their license being BSD-3)
I couldn’t find any communities still active on the site to test.
I don’t understand what you mean here, when I browsed the link that I previously shared I saw a respectable amount of communities with 200 to 300 current online users, which seems pretty active to me. Many if not most (if not all, as it is the target) are open-source projects, like Apollo Graphql which recently made the switch.
An additional argument is that Spectrum is likely to be integrated straight into Github at some point, as stated in the first link of this post (first acquiring the company): 3. Expect deeper integration with GitHub!
Most ROS packages being currently hosted on Github, if deeper integration means that each existing project can easily create its own community (or actually whatever else that means) it has the potential to interface better the ROS core platforms to the individual packages.