This is a release candidate for the 2.52.0 release. Packagers are requested not to update the main unison package to it (2.51.5 is still the most recent proper release), but are encouraged to package unison-snapshot or some similar name so that users can test this.
See the 2.52.0 entry at https://github.com/bcpierce00/unison/blob/master/NEWS.md
The major changes are a new wire protocol, a new archive format, and feature negotiation. While this release is expected to work seamlessly with existing archives and with 2.51.[0-5] peers, backups are always in order. Once this release has been run on a root, the upgraded archives will no longer be readable by stable 2.51 releases.
All users are encouraged to test this release candidate, and to begin planning how they will upgrade all computers running unison to it once 2.52.0 is formally released.