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.
I'm not disagreeing. But i think the difference is that the "left the left" cliche is a right wing trope used by media grifters. They didn't change, they rebranded. Because they aren't activists or comrades, they're a product.
People out of the media parroting parts of that trope like, "I'm as left as they come..." or "the left has gone too far" deserves a lot of skepticism. Not because people can't change, but because if they're talking like that today, they're taking that language from reactionary media.
Obviously that doesn't apply to Cleaver or Davis