Shameless plug: I literally just pushed a change[0] to my online note-taking app[1] last night that makes it a "full" Zettelkasten (e.g. there are backlinks now!) There are also some other cool features like multi-parent nesting that (in my opinion) make it much cooler than just a Zettelkasten. The main downside for many in the HN community is that it's online-only at the moment.
"Online-only" is not a deal breaker for me, but closed source and the missing possibility to self host it kinda is. But I'm gonna give it a try nonetheless!
Thanks for the feedback! One of our goals is to have a robust API that anyone can build apps for so that you can integrate this with any sort of platform you want (and then obviously the client could be open source), but yes, otherwise it is closed-source.
[0] https://supernotes.app/changelog
[1] https://supernotes.app