As open components, we have the OpenDocument standard + signal protocol for E2EE + CRDTs for conflict resolution. No idea whether they’re compatible though.
As a product, Collabora Online is open and collaborative.
Collabra seems close. They do use ODF. And you can host you’re own server.
But they don’t seem to use E2EE. And the collaborative aspect doesn’t apear to be an open standard you can use with different software packages.
E2EE would be nice, but what’s your idea of open standard for collaboration as opposed to simply open source?
If we had multiple software solutions implementing the same ways of collaborating what would be gained / in what ways would they differentiate and still remain compatible?