'Cancel culture' is a thing - it's been the subject of academic scrutiny, even by left-leaning professors and academics on how it affects movements within the left. And you're right, it isn't a uniquely 'leftist' thing. But purity politics (whether the predecessor or the underlying philosophy of how 'cancel culture' operates - regardless of whether it's a 'thing' or not) and moral grandstanding are serious topics of concern and interest to political and moral philosophers, and sociologists too. There is balanced academic analysis both in condemnation and in praise of so-called 'cancel culture'. I think, however, this comes down to a measure of practicality among activists and what they want to accomplish. There are several good ways of being true to your ideals and unwavering, without 'callouts' and 'cancelling'.

Whether or not you support Stallman, whether or not you find the accusations of 'cancel culture' in some places misguided and unfocused, to say that activism hasn't taken a turn to moral purity and the impossibility of forgiveness betrays either an ignorance of modern activism (which I consider myself a part of), or non-involvement in how these processes form. Arguably, 'cancel culture' also forms part of the carceral turn of various activist projects.

When the worst thing you can do is admit a mistake and apologize, there's a problem. When most of the population, according to polls, says it is reluctant to make its true political views known (right or left) - there's a problem.