Was it a major version upgrade? Postgres WAL replication is a physical replication stream which doesn't work across major versions. I don't know much about RDS

Was it a major version upgrade? Postgres WAL replication is a physical replication stream which doesn't work across major versions. I don't know much about RDS Postgres but I'd assume this is the reason. Logical replication can solve this, but has some important limitations (e.g. no replication of DDL), so it's understandable that RDS can't leverage this generically and automatically. I would expect that RDS MySQL doesn't have this problem since MySQL's built-in replication is logical rather than physical.