What makes you think that in RDS you are part of a "shared cluster"? I believe that with RDS, your database instance runs on EC2 instance(s) dedicated to you.
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Conce...
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Conce...
Shared or not is absolutely an assumption on my part. If they're spinning up an individual cluster for you, then some of the restrictions on RDS don't make much sense to me. They make more sense if it were shared.
Unfortunately, for most of the clients I work with, the limits (shared or not) are show-stoppers so I've only dealt with building a system backed by AWS RDS/PostgreSQL once and the need to dive into the underlying mechanics just wasn't there at the time.
The limitations are presumably so that they have control over settings for things like replication and their features that you can't mess with.