I just checked in the first go at this:
https://github.com/discourse/discourse/commit/92e371f0b38de9df495aa3f074177dc4251a205b
This is a the simplest possible implementation.
You can add a user to a list and then get no green or blue bubbles from them.
I have not implemented the site setting to notify the “mutee” on PMs they make, and honestly am not sure its worth adding, for one it complicates the code a fair bit.
I have not yet added the special CSS styling and intend to do so next.
