We maintain a stable LTS release (v2.0) for those who cannot tolerate breaking changes.

Feature releases (e.g. 2.1, 2.2) include new features and do break compatibility on occasion (so we can quickly introduce new behavior). Any breaking changes are well-documented in the release notes[1], so you can have a smooth upgrade path.

Minor releases (e.g. 2.1.1, 2.1.2) do not have breaking changes, only bugfixes and minor improvements.

I just opened an issue so we can document this more clearly: https://github.com/rethinkdb/docs/issues/948

[1] https://github.com/rethinkdb/rethinkdb/releases

I don't think there are actually any breaking changes. The release notes say "includeInitial" is breaking, but I think all of your existing application code will continue to run the same if you upgrade RethinkDB. It's just that new code using includeInitial will fail on old versions of RethinkDB.