The MacRumors article doesn't mention it, but the 11.6 and Catalina 2021-005 Security Updates supposedly address a zero day that may be actively being exploited (at least on Big Sur), so it's highly recommended you get all your Macs up to date as soon as possible. We're doing an emergency push to all our Macs starting this evening.
We already restrict iMessage and do not allow the use of AppleID's. I find it a bit strange they did not just roll this in to 11.5.3 or something to get it out faster and less impactful. They could have updated iMessage without rebooting the Macs with a much smaller update like they do with Safari.