Release notes (part of the migration guide): https://www.openssl.org/docs/man3.0/man7/migration_guide.htm...

Not too many breaking changes per-se, but lots of old API calls deprecated (pretty much all the old low-level APIs), plus new default values for a variety of things.

Also, going forward they'll be using the standard semantic version-numbering system.

If backwards compatibility is broken in 3.0 (it is), I would rather have a migration to openbsd's libressl.

A lot of distros migrated away from libressl (gentoo, alpine and void, iirc) because libressl is also not a drop-in replacement for openssl 1.1.x, or at least wasn't in 2020. The situation is not expected to change.