Exclude posts with specific meta_value while sorting by a separate meta_value

You code may be

$paged = (get_query_var('paged')) ? get_query_var('paged') : 1;

$args = array( 
    'posts_per_page' => '50',
    'meta_key' => 'release_date',
    'orderby' => 'meta_value_num',
    'order' => 'ASC',
    'meta_query' => array(
        array(
            'key' => 'undetermined_date_toggle',
            'value' => 'true',
        )
    ),
    'paged' => $paged
);
$the_query = new WP_Query( $args );

this code show only those posts which have undetermined_date_toggle key==true
or has checkbox check (selected) sorted by release_date_key.

Important Link:

WP_Query

*In meta_query parameter compare and type as your requirement not necessary to pass they have their default values = and CHAR, for more info read WordPress codex for WP_Query