Ultimately, I think right-leaning politicians want to ban deplatforming because deplatforming works and they have the most to lose from it. I see this as practical political calculus and a fight for survival rather than some ideological exercise.
A prominent politician does not lack means for getting their message out. If anything, being banned from a social media site provides free airtime on conventional media as they talk about it. It's unclear whether the net effect is positive, negative, or otherwise.
But it is clear that riling up people with talk about "deplatforming" is pretty effective. And, at least for the past couple of decades, riling up their constituency has been a fairly effective political tactic.
If deplatforming worked, the LGBTQ rights movement would have failed because they were kept in the closet. Deplatforming does not work, particularly in an era where "The 'Net interprets censorship as damage and routes around it."
I also assert that "The 'Net" of today is a much different one than that inspired John Gilmore's quote, as non-censoring alternatives to Twitter, Facebook, and YouTube have failed to gain much traction.
If "deplatforming" is taken to mean "depriving people of an easy means to find and communicate to an audience" it can be an effective technique depending on context, alternative communication channels, etc.
If "deplatforming" is taken to mean "a novel social ill based on depriving people of their right to communicate" then yeah it's a myth.
Free speech is like encryption at this point. Sure, it's not technically illegal, but if you actually try to do something for the public to use then the system will find some way to make it impossible and shut you down.
I admit that I also enjoy riling up first amendment fundamentalists but it isn't my main goal and these are my sincerely held beliefs so I don't think really qualifies as trolling.
Putting the potential straw man aside, wouldn't you say that your definition of a 'myth', whatever it might be, is ambiguous to the point where you can accuse anyone of spreading myths and disinformation, even if they're factually correct? For example, let's get everyone offended and say that someone could say that George Floyd getting killed by police is a myth, because he was killed for resisting and being a criminal, and it wasn't a bad thing at all. People cast a moral judgement on this event, so in a way it is a myth, is it not? Or is it when it's something that contradicts your own moral judgement?