Walid because there is no is:hidden filter on the post filterer.
Here are the default filters (author:, discussion:, id:, number:, type:) flarum/frameworktree/v1.8.9/framework/core/src/Post/Filter
You could create a "hidden" filter and register it using the PHP extension API.
Just to make sure this is understood: the is:hidden filter on the discussion list just removes discussions that are not hidden from the result. Hidden discussions and posts are always included by default, depending on whether the API user has permission to see them. You can always loop through all results (discussions or posts) and look for discussions or posts that have the hidden attribute.
