technically yes. they just recently made the sync server open source - https://github.com/streetwriters/notesnook-sync-server - but their documentation for it is still pending.

I’ve been following their progress for a while and can say that they appear to be following through on all their goals. and are very responsive to issues on GitHub. but don’t take my word for it, check out their roadmap to see when they release the self hosting documentation- https://notesnook.com/roadmap/