Show a message when it's an expired event. Event Espresso 4.

expired-event-message.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

add_action(
    'AHEE_event_details_after_event_date',
    function($post) {
        $event = EEH_Event_View::get_event($post->id);
        $status = $event instanceof EE_Event ? $event->get_active_status() : '';
        if ($status == 'DTE') {
            echo '<h4>This event is expired and not part of the 2020 schedule.</h4>';
        }
    },
    10
);