Cool project! Also check out rich text support in OutlineEdit 3: https://outlineedit.com/ It has an embedded plain text representation of all rich text formatting that is based on html-style tags and can mix and match all combinations of formatting styles. Also supports macOS versions back to 10.14 Mojave.

OutlineEdit 3 supports macOS 10.14 and newer, including Catalina: https://outlineedit.com/home.html

OutlineEdit 3 supports macOS 10.14: https://outlineedit.com/home.html

Just FWIW, I tried it.

It does work, but it's a little clunky IMHO. For instance, if you press Return in the middle of a line, I expect it to split the current paragraph into 2, but not in OutlineEdit (or Outlinely or OmniOutline).

OutlineEdit 3 supports macOS 10.14: https://outlineedit.com/home.html

OutlineEdit 3 is a great choice for a modern, native Mac outliner: https://outlineedit.com/home.html

I am the developer and always looking for feedback on the app. So let me know any thoughts!

And for those who care: It also supports every macOS version back to and including 10.14 (Mojave)