I'm almost positive GMail scanning messages is one cause. My domain got put on the list for a URL that would have been unknowable to anyone but GMail and my sister who I invited to a shared Immich album. It was a URL like this that got emailed directly to 1 person:
Then suddenly the domain is banned even though there was never a way to discover that URL besides GMail scanning messages. In my case, the server is public so my siblings can access it, but there's nothing stopping Google from banning domains for internal sites that show up in emails they wrongly classify as phishing.
Think of how Google and Microsoft destroyed self hosted email with their spam filters. Now imagine that happening to all self hosted services via abuse of the safe browsing block lists.
https://photos.example.com/albums/xxxxxxxx-xxxx-xxxx-xxxx-xx...
Then suddenly the domain is banned even though there was never a way to discover that URL besides GMail scanning messages. In my case, the server is public so my siblings can access it, but there's nothing stopping Google from banning domains for internal sites that show up in emails they wrongly classify as phishing.
Think of how Google and Microsoft destroyed self hosted email with their spam filters. Now imagine that happening to all self hosted services via abuse of the safe browsing block lists.