filter search result with custom post type meta key

This might not have worked before, but WordPress’s meta queries now support the relation parameter. To complete what @s_ha_dum was looking to do, simply add the relation parameter (as OR) and add a second meta query for NOT EXISTS. Example:

function searchExcludeOldMatches( $query ) {
    //if we're on the admin panel, or it's a search, or it's not the post type we want to filter, return the original query
    if( is_admin() || !$query->is_search() || $query->get( 'post_type' ) != 'yourposttype' )
        return $query;

    //this is the query we want to update
    $query->set('meta_query', array(
        'relation' => 'OR',
        array(
            'key' => 'cpt-match-date'
            ,'value' => date('Y/m/d',time())
            ,'type' => 'DATE'
            ,'compare' => '>'
        ),
        array(
            'key' => 'cpt-match-date'
            ,'value' => date('Y/m/d',time())
            ,'type' => 'DATE'
            'compare' => 'NOT EXISTS'
        ),
    ));
    return $query;
}
add_filter('pre_get_posts','searchExcludeOldMatches');

For further reading, check out the WP Meta Query Codex Page: https://codex.wordpress.org/Class_Reference/WP_Meta_Query

Edit Nov 17, 2015: Added filter for custom post type and simplified the conditional statement.