You'll never see true support for horizontal scalability in Postgres because doing so would require a fundamental shift in what Postgres is and the guarantees is provides. Postgres is available and consistent. It cannot truly be partitionable without impacting availability or consistency.
When an application grows to such a scale that you need a partitionable datastore it's not something you can just turn on. If you've been expecting consistency and availability, there will be parts of your application that will break when those guarantees are changed.
When you hit the point that you need horizontally scalable databases you must update the application. This is one of the reasons that NewSQL databases like CockroachDB and Vitess are so popular. They expose themselves as a SQL database but make you deal with the availability/consistency problems on day 1, so as your application scales you dont need to change anything.
Context:
I've built applications and managed databases on 10's of thousands of machines for a public saas company.
When an application grows to such a scale that you need a partitionable datastore it's not something you can just turn on. If you've been expecting consistency and availability, there will be parts of your application that will break when those guarantees are changed.
When you hit the point that you need horizontally scalable databases you must update the application. This is one of the reasons that NewSQL databases like CockroachDB and Vitess are so popular. They expose themselves as a SQL database but make you deal with the availability/consistency problems on day 1, so as your application scales you dont need to change anything.
Context: I've built applications and managed databases on 10's of thousands of machines for a public saas company.