Add a timestamp column and change the sort order to the Attendee Checkin admin page. Event Espresso 4.

checkins.php

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

add_filter(
    'FHEE_manage_event-espresso_page_espresso_registrations_columns',
    'my_filter_registration_list_table_columns',
    10,
    2
);
add_action(
    'AHEE__EE_Admin_List_Table__column_timestampcolumn__event-espresso_page_espresso_registrations',
    'my_registration_list_table_timestampcolumn', 
    10, 
    2
);
add_filter(
    'FHEE__Registrations_Admin_Page___get_orderby_for_registrations_query',
    'my_change_reg_checkin_orderby',
    10,
    2
);

/**
 * this function adds the column name to the array of table headers
 *
 * @param array $columns
 * @param string $screen
 * @return array
 */
function my_filter_registration_list_table_columns( $columns, $screen ) {
    if ( $screen === "espresso_registrations_event_registrations" && isset($_REQUEST['DTT_ID']) ) {
        $columns = EEH_Array::insert_into_array(
            $columns,
            array( 'timestampcolumn' => 'Checked In' ),
            'ATT_fname',
            false
        );
    }
    return $columns;
}

/**
 * this function echoes out the data you want to appear in your custom column.
 *
 * @param \EE_Registration $item
 * @param string           $screen
 */
function my_registration_list_table_timestampcolumn( $item, $screen ) {

    if ( $screen === "espresso_registrations_event_registrations" && $item instanceof EE_Registration && isset($_REQUEST['DTT_ID'])) {
        $DTT_ID = $_REQUEST['DTT_ID'];
        if (! empty($DTT_ID)
            && EE_Registry::instance()->CAP->current_user_can(
                'ee_read_checkins',
                'espresso_registrations_registration_checkins'
            )
        ) {
        $timestamps = $item->get_many_related('Checkin', array(array('DTT_ID' => $DTT_ID)));
            if (! empty($timestamps)) {
                // get the last timestamp
                $last_timestamp = end($timestamps);
                // get timestamp string
                $timestamp_string = $last_timestamp->get_datetime('CHK_timestamp');
                echo $timestamp_string;
            }
        }
    }
}

/**
 * this function changes the sort order of the reg checkins page
 *
 * @param array $orderby
 * @param array $data
 * @return array
 */
function my_change_reg_checkin_orderby($orderby, $data) {
    if(
        (!isset($data['action']) || isset($data['DTT_ID'])) &&
        isset($data['event_id']) &&
        isset($data['page']) &&
        $data['page'] == 'espresso_registrations'
    ) {
        $orderby = array();
        $orderby['Checkin.CHK_timestamp'] = 'ASC';
    }
    return $orderby;
}