The Modules Mirror and the Checksum Database are governed by this specific privacy policy:
https://proxy.golang.org/privacy"go env -w GOPRIVATE=*.corp.example.com" was added to make it as easy as possible to configure private modules. If the environment is not set up, just the name of the module will reach the Google services, it will not be published, and an error will be returned.
Anything else would have make it possible to bypass the protections of the new Checksum Database (https://golang.org/design/25530-sumdb), a state-of-the-art authentication system that provides auditable proof that the operator is behaving honestly.
For an example of why the Mirror is valuable, beyond the 10x performance gains (depending on number of dependencies and connection latency) and security gains from not using git on the client, look no further than git.apache.org, which has been down for days breaking a number of builds (https://status.apache.org/incidents/63030p4241xj).
I really think this architecture brings together the best of the decentralized module ecosystem that Go had (with its lack of regularly compromised registry accounts, and no mismatch between code and package origin) and the centralized registries of other languages (with their better performance and reliability).
Particularly proud of how the Checksum Database provides even more security than the centralized registries (because it's cryptographically accountable for its actions, while registries can sign anything they want) without requiring authors to manage keys, register an account, or change their workflow at all.
[Note that I work on the Go team, and co-designed the Checksum Database. But of course I don't speak for the company.]
EDIT to add: I should also mention that the sumdb allows the use of any untrusted proxy, easily configured with "go env -w GOPROXY", for users that can't or don't want to use the Google provided one. The proxy can also opt-in to mirroring the Checksum Database, in which case no user requests at all need to reach the Google services, while still providing all the integrity guarantees that proxy.golang.org users benefit from.