AFAKIT the built-in backup of a managed database will be gone if the database is deleted. This is true in AWS and GCP.

I still don't know why the product manager would decide this is a good UX.

IIRC in AWS you have the option to create a "final" snapshot of the DB instance when deleting it. I'm pretty sure that's the default behaviour when using the web console, but may merely be an option in the API interface.