Please point me to where GoDaddy or any other hosting site mentions public suffix, or where Apple or Google or Mozilla have a listing hosting best practices that include avoiding false positives by Safe Browsing…

>GoDaddy or any other hosting site mentions public suffix

They don't need to mention it because they handle it on behalf of the client. Them recommending best practices like using separate domains makes as much sense as them recommending what TLS configs to use.

>or where Apple or Google or Mozilla have a listing hosting best practices that include avoiding false positives by Safe Browsing…

Since were those sites the go to place to learn how to host a site? Apple doesn't offer anything related to web hosting besides "a computer that can run nginx". Google might be the place to ask if you were your aunt and "google" means "internet" to her. Mozilla is the most plausible one because they host MDN, but hosting documentation on HTML/CSS/JS doesn't necessarily mean they offer hosting advice, any more than expecting docs.djangoproject.com to contain hosting advice.

The underlying question is how are people supposed to know about this before they have a big problem?

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Nothing in this article indicates UGC is the problem. It's that Google thinks there's an "official" central immich and these instances are impersonating it.

What malicious UGC would you even deliver over this domain? An image with scam instructiins? CSAM isn't even in scope for Safe Browsing, just phishing and malware.