“Postgres 11.9 is being retired by Amazon’s RDS service on February 29, 2024.”
Knock ran Postgres 11.9 from day one and jumped to 15.3, four major versions, using logical replication instead of an in-place upgrade. An in-place upgrade on RDS means a shutdown that can run from minutes to hours, then VACUUM and REINDEX before the database is fully usable. Skip the work and Amazon forces the upgrade on its own date unless you sign a long-term support contract. Managed databases manage your calendar too.