Hey Feike,
This seems like an excellent Feature Request to make in the
Public Issue Tracker. I tested and confirmed the public schema is owned by cloudsqladmin, and of course that account has a
randomly generated, unique instance-specific password and used only by the infrastructure to install updates. We monitor the Public Issue Tracker regularly and you should see a quick result if you clearly explain the nature of the issue.
Now, I'm not an expert in Postgres but you might consider looking into whether it's possible to put users into a group / role that doesn't have access to the schema?
At any rate, feel free to reply here if you have any additional comments or questions.
Cheers,
Nick
Cloud Platform Community Support