It's possible that people can actually be former leftists/socialists/communists/etc and have "left the left" so to speak. I've been noticing here that the common response to this will be "they were never really leftists/socialists/etc. anyways" or "they were never really serious to begin with." It's possible that it could be the case, or maybe they actually were on the left, but they just didn't have the stomach for the real shit I guess.
I think we shouldn't be too quick to jump to using the "no true scotsman" fallacy in these cases. For those who don't know, it's basically saying "you weren't REALLY a leftist, cuz otherwise you never would have left." The same argument is used by religious people when someone leaves the faith: "oh they were never a REAL christian, otherwise they wouldn't have become atheist/agnostic."
To give some concrete examples, take Eldridge Cleaver. He actually was a Black Panther, radical, communist, etc. but then later in life became a Reagan conservative. Or Angela Davis. She actually was a radical but now she tells people to vote for Biden.
TL;DR: People can and do change, sometimes for the better, sometimes for the worse.
Are people saying that Angela Davis or Eldridge Cleaver were never actually on the left, or are you using an example outside of the normal scope of when public figures make a show of "Leaving the Left"?
Because in my experience, this discourse is almost exclusively around influencers and online figures, or primarily electoral figures. If you have a good example of someone like that being a pretty unambiguously genuine communist/socialist before turning to the right, then that would be more effective.