I've seen a couple of these "distributed" postgres extensions.

My question is, has any of them been talked about being upstreamed to postgres itself? Or, adding a custom built in feature to postgres itself?

This is not an extension, it's a proxy! Very different. You can deploy it anywhere already without having to wait for upstreaming or your cloud provider adding support for it. It's one of the two reasons why we built it this way, the other being performance (it's much faster to do this in the proxy than inside Postgres).