I've never heard of this perhaps because HN is about the only social media I use. However, there is an ADL page on this. See https://www.adl.org/education/refer

I've never heard of this perhaps because HN is about the only social media I use. However, there is an ADL page on this. See https://www.adl.org/education/references/hate-symbols/echo https://www.adl.org/education/references/hate-symbols/echo