Since 2020 I built https://sequencer.party

Supports third-party plugins in a format called WAM2 (Web Audio Module version 2).

Collaborative, like Google Docs. Sign in and start a session, invite people. Turn a knob, and it turns for everyone.

Supports audio, midi, and video as well.

Supports custom Javascript sequencers (Function Sequencer plugin). You can (collaboratively) write a little javascript program, give it knobs dials and automateable parameters, and have it emit MIDI events.

There's some usability issues to still work on but it's quite fun to jam on. I used to host twitch sessions where people could sequence my TR-909 and minimoog, and then hear the instruments played back over twitch.