The RFC is very clear about how extensions are supposed to be registered with IANA, which is always how RFC extensions in general work. You cannot have interoperability without a central registry.
IMO, many folks want SCIM with to support only two providers: Azure AD/Entra and Okta.
I guess there's a third: a homegrown system an enterprise has that "supports SCIM". That one is always going to be weird.
So in reality those two vendors get to determine acceptable behavior for SCIM servers (the data stores that push data into SCIM clients like Tesseral).