The only reason an authorisation server would support dynamic client registration, however, is because it's meant to be pluggable as the back-end for an API integration suite like Kong Enterprise.
The solutions that are one big well-known IdP have no reason to want it, so Google Auth, Amazon Cognito, and similar all don't support it.
The solutions that are aimed at letting organisations be an IdP do have reason to offer it: it makes being an IdP easier, because you can stand up a client registration service without using vendor-specific interfaces.
No-one who operates an IdP wants arbitrary, uncontrolled public client registration. It'll get abused directly, and it'll enable further abuse of the systems the IdP is meant to protect.
https://swagger.io/docs/specification/authentication/openid-...