At first I thought it was only Java edition but now it seems that all platforms will have it. We have to upload things like Digital ID or other methods to Yoti, a third party company, to verify our age if we want to use social features such as the chat. This means if we don’t even in single player the chat will be removed. Then we can’t even use coords or commands in Java single player. As of now it is unclear how and when exactly they will add this but they said on their website it is next month sometime. Not sure if it will be online only and if it will be a certain update or all updates. I doubt mods will be able to get around this. Mojang will probably tell them to be removed.
That’s gonna be impossible to block, as plugins can just add their own commands /msg /send /w /etc. So this will either fall on the plugin developers having to ‘mark’ their custom commands, be very impossible to enforce or command usage will be blocked