Support for the Isabelle release process, see also the blog.
Main repository fork (read-only): https://isabelle.sketis.net/repos/isabelle-release
Isabelle releases are time-based: every 8-10 months the repository state is consolidated into a stable version that can serve end-users for a few years. That process typically takes 4-8 weeks, with 3-5 public release candidates. After a release is finalized, there are no further updates nor "patches'' to it.
When the release train is about to depart, only finished and consolidated features will make it on the trip. Anything that is not properly working will be postponed to a future release (deactivated or just not announced).