The release announcement threads contain a list of what is new with each release. Scroll to the end of each to see the latest.
Rhino 7 Service Releases: Rhino 7 Service Release Available - #12 by brian
Rhino 7 Service Release Candidates: Rhino 7 Service Release Candidate Available - #31 by brian
Rhino 8 Work In Progress: Rhino 8 WIP Available Now! - #18 by brian