Andy (the Orbit author) has his own very interesting Zettelkasten-like set of public notes. In this page, for example, he compares/contrasts his "Evergreen note" approach to a Zettles in a Zettlekasten: https://notes.andymatuschak.org/z4AX7pHAu5uUfmrq4K4zig9x8jmm...

Additionally, his system does have spaced repetition built-in, too:

"I’ve embedded a Spaced repetition memory system into my notes: The mnemonic medium can be extended to one’s personal notes. This has substantially changed the medium, but in ways I don’t yet understand very well."

I do not believe he has open-sourced his note taking system, but it is fun to browse around in; he presents a lot of interesting ideas, particularly if you're into this space.