There are several forks of PostgreSQL, in various levels of license, additional features and activity. However, maintaining a fork in addition to a main project is inherently more expensive than maintaining just a single project, so adding features to new major releases of the main project is generally preferred over forking every release into its own, newly named, project. After all, that is what we have major (feature) releases and stabalization windows (beta releases) for.

This won't work well for a multiyear project.. Either you have to stall the release process, divide it into smaller parts or fork.