I bet some of those who advocated "it's a private company, it can deplatform whoever it wants" are probably re-thinking their stance on deplatforming from a major social media.
Everything that follows is based on that inference.
There are two major possibilities here. Either Musk lets twitter continue much in the same manner as it operates today. In which case, nothing really changes.
Musk operates twitter as his personal bullhorn, promoting beneficial tweets and removing tweets that are critical of him, his companies, his ideals, whatever. All of which would be his right to do. However, all actions have consequences.
Twitter isn't twitter because it was ordained from on high as the service to spit hot takes in 140 characters or less. Twitter is twitter because of everything twitter did. From the initial concept, to the pivoting to whatever the hell "micro-blogging" is, to letting people say mostly whatever, to the various standards they've implemented over the years. All of that makes twitter what it is.
If twitter just becomes "Elon Musk's dream platform where he can shit post all day and no one can say shit to him", that's entirely something else. Most of the people left on twitter would be Musk and people who want to follow him.
Like how the crowd shifted from Slashdot to Digg to reddit. From Friendster to MySpace to Facebook. Something will come along and supplant twitter.
So, I believe the people who "advocated "it's a private company, it can deplatform whoever it wants"" are going to be fine with their stance. Because if Musk changes twitter into a shithole, no one is going to want to stay regardless.