YugabyteDB is also a fork of PostgreSQL. Last year, they got around to rebasing from 11 to 15, and spent some time working on better isolating their changes to make that easier going forward.
> When we set out to make YugabyteDB Postgres-compatible, we took a fork of Postgres, and modified all of the operations that use shared memory or storage to instead talk to our LSM- and Raft-based distributed storage and transaction layer.
These are forks or extensions of Postgresql.