Exclude post by custom meta with pre_get_posts

Question

Your can use the “meta_query” argument of the query, I think is better than filter the post_where:

add_action( 'pre_get_posts', 'bbwp_calendar_visible_events' );
function bbwp_calendar_visible_events( $query ) {

    $postType="bbwp_calendar";

    if ( !is_admin() && $query->is_main_query() ){

        if ( is_post_type_archive( $postType ) ) {

            $meta_query = array(
                              'relation' => 'OR',
                              //current time <= event_end_date
                              array(
                                  'key'   => 'event_end_date',
                                   //Check https://codex.wordpress.org/Function_Reference/current_time
                                   //to return the current time (today) in the same format
                                   //you store the date in your custom field
                                  'value' => current_time( 'timestamp' ),
                                  'type' => 'DATETIME',
                                  'compare' => '<=',
                              ),
                              //event_end_date=""
                              array(
                                  'key'   => 'event_end_date',
                                   //Check https://codex.wordpress.org/Function_Reference/current_time
                                   //to return the current time (today) in the same format
                                   //you store the date in your custom field
                                  'value' => "",
                                  'type' => 'DATETIME',
                                  'compare' => '=',
                              ),
                     );

            $query->set( 'meta_query', $meta_query );

         }
     }
}

More info: