This will ensure that the old appointments (the time of which has passed) are deleted daily.

delete-old-appointments.php

<?php

add_action( 'init', function() {
    
    if ( ! function_exists( 'jet_apb' ) ) {
        return;
    }
    
    if ( ! wp_next_scheduled( 'delete_old_appointments' ) ) {
        wp_schedule_event( time(), 'daily', 'delete_old_appointments' );
    }
    
    add_action( 'delete_old_appointments', function() {
        
        global $wpdb;
        
        $table = jet_apb()->db->appointments->table();
        
        $wpdb->query( "DELETE FROM $table WHERE slot_end < UNIX_TIMESTAMP();" );
        
    } );
    
} );