I think it's too complex to attribute to one thing, I think you're right that the alt right social media pipeline is real and an issue.
I'm just saying I understand not identifying with a group who trash an identity a person belongs to. They just disengage. Nobody likes being put into a box that paints them as a villain.
Just my opinion of course, the left (as a social group) can examine the current discourse and try to adjust, maybe like framing it as a class struggle or a liberty struggle, or continue being divisive and risk being overridden.
I'm honestly not sure it's even vindictive. Maybe in some cases, it's a big complicated issue.
We get such different news in the age of algorithms, they may not even see it as vindictive. I think it's easy to pull back from a group that paints your identity as a villain, and I think that's likely some of it.