CORSでSimple Requestになる条件としてMDNに下記の記載があるのですが、“forbidden header name”に記載されているヘッダ（例えば"Cookie"）はSimple Requestになるのでしょうか？

Apart from the headers set automatically by the user agent (for example, Connection, User-Agent, or any of the other headers with names defined in the Fetch spec as a “forbidden header name”), the only headers which are allowed to be manually set are those which the Fetch spec defines as being a “CORS-safelisted request-header”, which are:

https://developer.mozilla.org/ja/docs/Web/HTTP/CORS