FBI has been engaged with subterfuge against dissident groups since its inception. With modern left leaning groups this started in the 60s-70s. Domestically in America this was with ops like COINTELPRO in the wake of the civil rights movement, internationally it was with ops like Operation Condor to squash left-leaning governments in the western hemisphere. I point this out to say modern leftism isn't something that has been defanged merely on account of inheriting its baggage from "the 1700s", but as a process with particular events that can be understood.
Now I'm not gonna say left ideologies were outright shadowfunded by government agencies but, along with the above, the CIA/FBI *did* read and keep up with "left" theory like feminism and post-modernism (inasmuch as government agencies kept up with previous developments in social sciences like Edward Bernay's development of propaganda borne out of psychology, put to effect by WW2 among many nations). If those government agencies saw the identity politics latent in those ideas that could obfuscate material or class politics, they certainly would not be opposed to the potential of leftist self-cannibalization by battle lines being drawn in terms of race and gender, rather than class and material reality (ie traditional Marxist theory).
From an optics perspective, the issue today is that left material politics gets conflated with "left" (read: liberal) identity politics, poisoning the well of discourse from every perspective. I'm not going to say there was a grand conspiracy to do this but I note the structuring of popular optics certainly doesn't facilitate leftism being properly represented in culture (articles about gender or race will get the greenlight at publications much more eagerly than those deemed "class reductionist"), which ultimately leads to its perception in the average person's psyche and how unpalatable it is for those people it (seemingly) doesn't reach out to.