Say what you want it's a recommendation and it's documented in quite a few deployment methods. The only benefit of centralizing it is if you are managing postres without other tools since it'd be a pain in the butt. You'll still run into apps that doesn't run on later versions and others that require later versions though.

An example of a very popular one:

How many databases should be hosted in a single PostgreSQL instance?

Our recommendation is to dedicate a single PostgreSQL cluster (intended as primary and multiple standby servers) to a single database, entirely managed by a single microservice application. However, by leveraging the "postgres" superuser, it is possible to create as many users and databases as desired (subject to the available resources).

The reason for this recommendation lies in the Cloud Native concept, based on microservices. In a pure microservice architecture, the microservice itself should own the data it manages exclusively. These could be flat files, queues, key-value stores, or, in our case, a PostgreSQL relational database containing both structured and unstructured data. The general idea is that only the microservice can access the database, including schema management and migrations.

CloudNativePG has been designed to work this way out of the box, by default creating an application user and an application database owned by the aforementioned application user.

Reserving a PostgreSQL instance to a single microservice owned database, enhances:

resource management: in PostgreSQL, CPU, and memory constrained resources are generally handled at the instance level, not the database level, making it easier to integrate it with Kubernetes resource management policies at the pod level
physical continuous backup and Point-In-Time-Recovery (PITR): given that PostgreSQL handles continuous backup and recovery at the instance level, having one database per instance simplifies PITR operations, differentiates retention policy management, and increases data protection of backups
application updates: enable each application to decide their update policies without impacting other databases owned by different applications
database updates: each application can decide which PostgreSQL version to use, and independently, when to upgrade to a different major version of PostgreSQL and at what conditions (e.g., cutover time)