Source

File: wp-includes/class-wp-xmlrpc-server.php

private function _toggle_sticky( $post_data, $update = false ) {
		$post_type = get_post_type_object( $post_data['post_type'] );

		// Private and password-protected posts cannot be stickied.
		if ( 'private' === $post_data['post_status'] || ! empty( $post_data['post_password'] ) ) {
			// Error if the client tried to stick the post, otherwise, silently unstick.
			if ( ! empty( $post_data['sticky'] ) ) {
				return new IXR_Error( 401, __( 'Sorry, you cannot stick a private post.' ) );
			}

			if ( $update ) {
				unstick_post( $post_data['ID'] );
			}
		} elseif ( isset( $post_data['sticky'] ) ) {
			if ( ! current_user_can( $post_type->cap->edit_others_posts ) ) {
				return new IXR_Error( 401, __( 'Sorry, you are not allowed to make posts sticky.' ) );
			}

			$sticky = wp_validate_boolean( $post_data['sticky'] );
			if ( $sticky ) {
				stick_post( $post_data['ID'] );
			} else {
				unstick_post( $post_data['ID'] );
			}
		}
	}