PHP

<div class="side_archive_menu">
    <?php
    $year_prev = null;
    $months = $wpdb->get_results("SELECT DISTINCT MONTH( post_date ) AS month ,
                                      YEAR( post_date ) AS year,
                                      COUNT( id ) as post_count FROM $wpdb->posts
                                      WHERE post_status = 'publish' and post_date <= now( )
                                      and post_type = 'post'
                                      GROUP BY month , year
                                      ORDER BY post_date DESC");
    foreach($months as $month) :
    $year_current = $month->year;
    if ($year_current != $year_prev){
    if ($year_prev != null){?>
        <?php } ?>
  <p class="toggle_btn"><?php echo $month->year; ?>年</p>
  <ul class="child">
    <?php } ?>
    <li>
        <a href="<?php bloginfo('url') ?>/<?php echo $month->year; ?>/<?php echo date("m", mktime(0, 0, 0, $month->month, 1, $month->year)) ?>">
            <?php echo date("n", mktime(0, 0, 0, $month->month, 1, $month->year)) ?>月
            (<?php echo $month->post_count; ?>)
        </a>
    </li>
    <?php $year_prev = $year_current;
    endforeach; ?>
  </ul>
</div>
<!-- /.side_archive_menu -->

抽出されるHTML

<div class="side_archive_menu">
	<p class="toggle_btn">2018年</p>
	<ul class="child">
		<li><a href="">11月(1)</a></li>
		<li><a href="">10月(14)</a></li>
		<li><a href="">9月(1)</a></li>
	</ul>
</div>
<!-- /.side_archive_menu -->

CSS

.child {
  display: none;
}

jQuery

$(".toggle_btn").on("click", function() {
     $(this).next().slideToggle();
     $(this).toggleClass("active");
 });

add_action(
	'init',
	function() {
		remove_post_type_support( 'ポストタイプ名', 'editor' );
	},
	99
);

<?php
/*
----------------------------------------------------------------------
widgetの設定があれば表示
----------------------------------------------------------------------
*/
if ( is_active_sidebar( 'sidebar1' ) ) {
	dynamic_sidebar( 'sidebar1' );
}
?>

<?php wp_get_archives( 'type=monthly&post_type=ポストタイプ名' ); ?>

<?php if ( has_post_thumbnail() ) : /* もしアイキャッチが登録されていたら */ ?>
<img class="lazy" src="<?php echo esc_url( $my_template_url ); ?>/img/dummy-lazy.png" data-src="<?php the_post_thumbnail_url(); ?>" alt="<?php echo esc_html( the_title() ); ?>">
<?php else : /* 登録されていなかったら */ ?>
<img class="lazy" src="<?php echo esc_url( $my_template_url ); ?>/img/dummy-lazy.png" data-src="<?php echo esc_url( $my_template_url ); ?>/img/dummy-photo.png" alt="<?php echo esc_html( the_title() ); ?>">
<?php endif; ?>