Overview#RFC 7009 and registered in the OAuth Parameters Registry.
Unsupported_token_type indicates the Authorization Server does NOT support the revocation of the presented token_type. That is, the client tried to revoke an access token on a server not supporting this feature. If the server responds with HTTP status code 503, the client must assume the token still exists and may retry after a reasonable delay. The server may include a "Retry-After" header in the response to indicate how long the service is expected to be unavailable to the requesting client.