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.
I like Discord, especially in the early days, you can reach out to the principal dev, etc. But it soon seems like they either disappear to get work done (good) or spend all thier time on it (bad). Either way you end up with chaos.
1. they're encouraged to do so in public, so that other community members can help if possible, and/or so bots can reply with suggested FAQ answers;
2. the Community Manager will answer with the company line for questions the company has set answers to (e.g. "when are you releasing X?" or "why is [abusive DoS-like pattern of requests to your service] not working?");
3. otherwise, if the Community Manager knows the answer for sure off the top of their head, they'll give the answer;
4. and if not, the Community Manager relays the question to an engineer in our Slack, where we either have an answer off the top of our heads, or we file it as an issue.
Seems to work just fine for us so far.
Some of the engineers are also sometimes in the Discord (and we're all registered to it), but other than the Community Manager, it's not our job to be in there.
Imo, this is not scalable or sustainable, and mailing lists are not a replacement for adequate customer support.
The only reason sending emails directly to mailing lists for specific Google products works is precisely because those mailing lists are not public and not flooded with bajillions of emails from the general public. So those who send the emails are already somewhat pre-screened in a way, because if you know that mailing list email address in the first place, you are very unlikely to send something like "my cousin couldn't remember password to their google photos account, can you fix this please". That's why everything there ends up being read and addressed. If those mailing lists were public, then they would be just as useless and ineffective as the current customer support routes currently are for Google.
Tl;dr: mailing lists for specific products are a nifty workaround for the time being, but they aren't a good sustainable solution for shitty customer support. Making those mailing lists public will not only not help solving the problem, it will just make those mailing lists as ineffective as the current customer support. There is no "one weird trick" to solve the customer support adequacy issues with Google,it has to be an actual customer solution that won't be easy and will take time.