I've seen it mostly on far left servers in US. Pleroma is Mastodon server software. For a bunch of essentially random reasons, it was popular among right-winge

I've seen it mostly on far left servers in US. Pleroma is Mastodon server software. For a bunch of essentially random reasons, it was popular among right-wingers setting up their Mastodon instances, and some servers responded by blocking any Mastodon server running it outright. A subset of those would also block any server not blocking Pleroma like they do.