A server receiving such a request and is unwilling to register MAY
silently ignore the registration request and process the GET request as
usual.
I think you mean:
If a server receives such a request and is unwilling to register
the observer client, the server MAY silently ignore the registration
request and process the GET request as usual.
From https://mailarchive.ietf.org/arch/msg/core/mBfPbB892W95BnAHEq9wtv8aH9Y/
The text says:
I think you mean: