A safe browsing service is not a terrible idea (which is why both Safari & Firefox use Google for this) & while I hate that Google has a monopoly here, I do think a safe browsing service should absolutely block your preview environments if those environments have potential dangers for visitors to them & are accessible to the public.
To be clear, the issue here is that some subdomains pose a risk to the overall domain - visiting any increases your risk from others. It's also related to a GitHub workflow that auto-generates new subdomains on demand, so there's no possibility to have a fixed list of known subdomains since new ones are constantly being created.
It is a terrible idea when what is "safe" is determined arbitrarily by a private corporation that is perhaps the biggest source of malicious behavior on the web.
1. You should host dev stuff and separate domains.
2. Google shouldn't be blocking your preview environments.