I'm confused about what you mean about "public." I'm just a regular guy with no connections to Google, other than being a GCP customer. I found the mailing list addresses for each GCP service listed directly in the support documentation. Literally anyone who has GCP problems would end up finding those addresses, if 1. they clicked on the "help" button and went through the workflow presented, and 2. didn't first pay for extended white-glove support and then immediately reach for it for any problem that came up.
By my thinking, that's a "public" mailing list. They're not hiding it from you. The opposite, really — they're trying to get everyone to know and use it, by making it free to any GCP customer, while the actual CSR kind of support requires paying for a subscription to a higher support tier. The mailing list, presented in Google Groups format, is literally what GCP calls their "support forum." It's supposed to take on all comers, including dumb customer asks.
By my thinking, that's a "public" mailing list. They're not hiding it from you. The opposite, really — they're trying to get everyone to know and use it, by making it free to any GCP customer, while the actual CSR kind of support requires paying for a subscription to a higher support tier. The mailing list, presented in Google Groups format, is literally what GCP calls their "support forum." It's supposed to take on all comers, including dumb customer asks.