functions.php

<?php

/**
 * List post headings 
 * @see https://www.billerickson.net/access-gutenberg-block-data/
 */
function be_list_post_headings() {
	global $post;
	$blocks = parse_blocks( $post->post_content );
	$headings = array();
	foreach( $blocks as $block ) {
		if( 'core/heading' === $block['blockName'] )
			$headings[] = wp_strip_all_tags( $block['innerHTML'] );
	}
	if( !empty( $headings ) ) {
		echo '<ol class="table-of-contents">';
		foreach( $headings as $heading )
			echo '<li>' . $heading . '</li>';
		echo '</ol>';
	}
}