Citus Data enables scale out, while also being a pure extension. That means you can upgrade Postgres like normal to the latest point release using whatever normal upgrade process you want (e.g. OS packages).

It has worked for a long time without the need for Postgres 12. However, the new APIs introduced in v12 did enable us to offer columnar compression as an option, which complements a lot of scale-out use cases.

See: https://www.citusdata.com/blog/2021/03/06/citus-10-columnar-...

I believe using the extension facilities of Postgres is far superior to a fork in the medium to long term.

(Disclaimer: I work for Citus, and on columnar compression.)