Obligatory mention of SubEthaEdit: "General purpose plain-text editor for macOS. Widely known for its live collaboration feature."

And, as of 2018, is now open source. Looks like it has had commits as recently as 2022: https://github.com/subethaedit/SubEthaEdit

See also a previous HN discussion at https://news.ycombinator.com/item?id=18550649