I have been reporting on far leftists, Antifa, and peripheral groups for nearly a decade (among others) and I have never seen the red triangle used by these groups

The only examples I've found are kind of similar but I really think this is weird shit https://www.washingtonpost.com/politics/2020/06/18/trump-campaign-runs-ads-with-marking-once-used-by-nazis-designate-political-prisoners/

Facebook has since banned the ad with the red triangle

The trump team argued it was used by Antifa

The left is arguing that Trump's campaign is "crop dusting" nazi iconography in order to trigger an over reaction on the left

"crop dusting" being that they get as close as possible without stepping over the line so that they get ads banned and can complain of bias

I'm not asserting that is true, just explaining what leftists are arguing