The parameter may not be getting passed back to MWSearch properly, or it may not be applicable there.

rainman wrote:

Listing/unlisting redirects is not applicable for MWSearch, and is probably just ignored. (when a redirect to a certain title matches it will be highlighted on a search page in parenthesis after the title)

(In reply to comment #2)

Listing/unlisting redirects is not applicable for MWSearch, and is probably
just ignored. (when a redirect to a certain title matches it will be
highlighted on a search page in parenthesis after the title)

Marking as INVALID, based on this comment and the assumption that it means that MWSearch doesn't return redirect pages. Other search engines may still return redirects, of course.