In order to provide a managed service arbitrary use of third party extensions has to be blocked. The RDS team tests and regularly adds support for extensions as it releases new versions, but there are many types of extensions that can't be supported. Sometimes that is because they aren't being sufficiently maintained. In the case of Multicorn there hasn't been a release since 2016, and hasn't had a check-in for 3 months, so there would be concern that if a major issue came up there would be no way for AWS to support the extension. In the case of a security issue, for example, the RDS team might have no option but to pull support for the extension with little or no warning to the customer.
Another reason extensions aren't supported is when they could allow access that interferes with database integrity, security, or RDS' ability to manage the database instance. For more information on the challenges in supporting arbitrary extensions (in a broader context of the challenges of having a managed service) see https://hal2020.com/2018/01/20/challenges-of-hyperscale-computing-part-2/ and https://hal2020.com/2018/08/25/challenges-of-hyperscale-computing-part-3/
Allright, thanks for the detailed response!
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