After confirming with a few team members, it should be possible now as our shipped builds are GPLv3 and so are the VST3 headers. It comes down to developing it. The main developer who worked on VST 2 support in OBS is quite busy and unable to work on VST 3 support at this time. We'd love to do it though. One person is looking into ways that would require less development and better compatibility (but comes with the downside of making OBS much larger), so we'll see how that goes.