This is an alpha release, and is a step towards a 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/f229ad2a2ebc547b9b6432046dda622eee4fc775/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.