ACF Custom Field WP_Query, but need to get all posts, if field doesn’t exist

Question

You can get both with an OR meta_query that also checks if the key does not exist:

'meta_query' => array(
    'relation' => 'OR',
    array(
        'key' => 'event_date',
        'compare' => 'NOT EXISTS',
    ),
    array(
        'key' => 'event_date',
        'value' => date("Ymd", time()),
        'compare' => '>',
    ),
),
'orderby' => array(
    'meta_value_num' => 'DESC',
    'date' => 'ASC',
),