PHP snippet to add a panel to the WP Block Inserter.

wclancpa-2019-block-categories.php

<?php
/**
 * Adding a block category creates a Panel
 */
function create_wclancpa_2019_panel( $categories, $post ) {
	return array_merge(
		$categories,
		array(
			array(
				'slug'  => 'wclancpa-2019',
				'title' => __( 'WC Lancaster 2019 Panel', 'wclancpa-2019' ),
			),
		)
	);
}
add_filter( 'block_categories', 'create_wclancpa_2019_panel', 10, 2 );