It's not the same thing, but it's still Google, soo.... all three of my kids are on school or club sports teams that use Teamsnap (https://www.teamsnap.com/) for coordination. This includes exposing calendar subscriptions to Google Calendar and delivering messages & event/change notifications via email (and push notification via the mobile app). For some reason, Gmail has decided that Teamsnap is no longer a trusted sender and throws big caution notices with every email. It's exceptionally annoying, and stupid, given what the app is, what it's used for, and how long it's been around. I don't know whether it's an ML heuristic that's picking up on something, whether it's an email server configuration change on the Teamsnap side, or whether Google's just created a bug, but it impacts potentially all of Teamsnap's 25 million users in the US.
So, after checking this thread again I went and searched for unread Teamsnap emails in my inbox again. None are displaying warnings now/anymore. This is after several days where all of them were, and since I posted this reply an hour ago the problem has ceased.
Reputation is crowdsourced. Perhaps at some point Teamsnap lost control of their infra and sent out a bunch of malware. Why do you assume the mistake was Google's? Same question for the OP. If Google was trying to tell me my site was compromised the first course of action I would take is to have a look into that, or page secops, before that situation spreads further.
After years of similar issues, why are you giving them the benefit of the doubt? Especially if they won't explain what the issue is and gave out such ludicrous advice to the OP?
There have been quite a few complaints along these lines on HackerNews and elsewhere. Sometimes Google gets it wrong, and then people end up stuck, as you can't contact them.
