Musk himself
posted the current Twitter Safety rules. I don't know how much has changed (maybe nothing yet?) but under the rules pretty much everyone currently banned such as Trump, Milo, Gavin McInnes and Alex Jones would remain banned.
As much fun as the parody accounts mocking Musk are, I agree that under the rules they should be banned, and I also agree with the principle of banning people who impersonate others. Not sure if that is really against the first amendment, I doubt it though. I am pretty sure the rule about parody accounts has been there for a while as I know of people posting even the most obvious parodies being told to clearly state they are parodies or risk being banned.