[@mediaformat Participant 4 days, 7 hours ago]

Unfortunately BuddyPress search doesn’t use WP_Query.

The best you can do with BuddyPress search is to use the bp_search_query_results filter.

function bp_exclude_from_search( $search_results ) {
    return array_filter( $search_results, function( $result ) {
        $excluded_post_ids = array( 1, 2, 3 );
        $excluded_post_types = array( 'custom_type', 'other_cpt' );
        if ( 
            ! in_array( get_post_type( $result->id, $excluded_post_ids ) ) 
            || ! in_array( get_post_type( $result->type, $excluded_post_types ) ) 
        ) {
            return true;
        };
    });
};
add_filter( 'bp_search_query_results', 'bp_exclude_from_search', 10 );