Amazon RDS now supports PostgreSQL major version 11 in AWS GovCloud (US) Regions. PostgreSQL 11 includes major improvements to partitioning, improvements to parallelism, and many other useful performance improvements like adding columns with a non-null column default faster. This version includes SQL stored procedures that allow embedded transactions within a procedure. 

from Recent Announcements https://aws.amazon.com/about-aws/whats-new/2019/08/amazon-rds-postgresql-supports-minor-version-114-for-aws-govcloud/