Google flags Immich sites as dangerous

If you're going to host user content on subdomains, then you should probably have your site on the Public Suffix List https://publicsuffix.org/list/ . That should eventually make its way into various services so they know that a tainted subdomain doesn't taint the entire site....

  In the past, browsers used an algorithm which only denied setting wide-ranging cookies for top-level domains with no dots (e.g. com or org). However, this did not work for top-level domains where only third-level registrations are allowed (e.g. co.uk). In these cases, websites could set a cookie for .co.uk which would be passed onto every website registered under co.uk.

  Since there was and remains no algorithmic method of finding the highest level at which a domain may be registered for a particular top-level domain (the policies differ with each registry), the only method is to create a list. This is the aim of the Public Suffix List.
  
  (https://publicsuffix.org/learn/)

So, once they realized web browsers are all inherently flawed, their solution was to maintain a static list of websites.

God I hate the web. The engineering equivalent of a car made of duct tape.

"The engineering equivalent of a car made of duct tape"

Kind of. But do you have a better proposition?