From Mr. Stephen Brennan on his blog:

What happened: for some reason, my site got onto a list called ‘Google Safe Browsing’, which is a list of sites which may contain malware or social engineering attacks. Browsers can check URLs against this list and warn users when they are going to an ‘unsafe site’. I don’t know much about it, and so I don’t want to make too many assertions about how it works. Instead, here’s my guess about why I got blocked: it seems like whatever automation was behind this classification saw that my site’s pages (especially the login page) looked similar to several other, more popular sites (i.e. other Mastodon instances), and so assumed that my site was phishing users.

I wrote an article last August about a site that had been de-indexed by Bing with no explanation, explaining why de-indexing by Bing is worse than it first appears because many “alternative” search tools, namely DuckDuckGo, Ecosia, and Qwant, are really Bing front-ends.  We were inexplicably blacklisted by Bing several months after I wrote the article about other victims of Microsoft’s arbitrary and capricious behavior (this behavior does not affect the Chinese Communist Party, however), and I can confirm that the horrors I described in my August piece are true.  At the risk of tempting fate, I will not write a full article about the dangers of being placed on Google’s safe browsing blacklist, but I can imagine, as the author describes in his piece, that this would be highly sub-optimal.