Gmail drafts are my #1 note taking app. Plain text, no formatting required but it's there if you want. Just hit Send to share a note. Not collaborative editing, but does solve all of the sync issues.
SyncThing is one of the most underrated programs out there.
>Sharing
There's a billion ways to share a text file with someone
>Collaborative editing
That's a fair point. Although, I've never had the need to collaboratively edit a text file. But it does look like there's stuff out there e.g. EtherPad, Gobby, HackMD.
Or when you want to share them and edit them collaboratively with someone else.