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.

https://www.yugabyte.com/blog/yugabytedb-moves-beyond-postgr...

> 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.