The title is misleading, the replication is not coming to stock Postgresql 9.6. A replication extension got the patches it needs to run into Postgresql 9.6 so you can use the extension without patching Postgres.
The purpose of extensions is they allow you to run stuff without including it in core database. What language should be used for this case to avoid confusion in future?
"Bidirectional Replication extension no longer needs patches in PostgresSQL 9.6" ?