If the argument that being antisemitic precludes you from being a leftist, that's not a fallacy. If you don't abide by a set of beliefs, then you are not a follower of those beliefs.

I'm not a Scotsman because I'm not from Scotland.

Similarly, I don't think most libertarians are true libertarians because they don't care about liberties except their own. And if that's the definition of libertarian, then everyone is a libertarian.

"No true leftist is antisemitic."

A strawman inventing a no true Scotsman? That's impressively fallacious 😄

I didn't and indeed have never claimed that there aren't any antisemites on the Left. Unfortunately, a lot of people on the Left are starting from the point of "the country (incorrectly) claiming to be representative of all Jewdom has been committing unspeakable atrocities for a century" and being misled from there down rabbit holes that sometimes go all the way to ACTUALLY antisemitic conspiracy theory nonsense.

That being said, those people are a tiny minority, and your two examples from the days of disco and earlier don't prove otherwise.