My company does managed Postgres on AWS, Azure, Google Cloud Platform, DigitalOcean and UpCloud - https://aiven.io/postgresql. Compose offers managed Postgres (https://www.compose.com/postgresql), as does ElephantSQL (https://www.elephantsql.com/). Database Labs is one more provider (https://www.databaselabs.io/).

Google recently added beta version of Postgres to Cloud SQL as well (https://cloud.google.com/sql/docs/postgres/). Of course, AWS RDS and Heroku have been on the market for some time.

I'd claim managed Postgres market is in a pretty good shape.