Further on this subject.
I believe that the Support channel should remain exclusively on Forum. Why? So all users can benefit and refer to it over time, using the handy search function. Matrix does not provide that.
A potential time delay (Matrix vs Forum) is not significant, since none us (with few exceptions) are glued to our screens anyway.
Off-Topic is ok on both Matrix and Forum, although again, more users would be involved if it too was kept on Forum only.
Development is for developers, so it is up to them if they want to stay on Matrix.