See https://twitter.com/chriscoyier/status/1164337392058134528

tags.php

<?php
class CTF_Register_Taxonomies {

	/**
	 * Initialize the class
	 */
	public function __construct() {
		add_action( 'init', array( $this, 'tags_support_all' ) );
		add_action( 'pre_get_posts', array( $this, 'tags_support_query' ) );
	}

	/**
	 * Associate tags with the Page post type
	 */
	public function tags_support_all() {
		register_taxonomy_for_object_type( 'post_tag', 'page' );
	}

	/**
	 * Ensure all post types that support tags are included in the main query
	 *
	 * @param WP_Query $wp_query
	 */
	public function tags_support_query( $wp_query ) {
		// Bail if the query isn't the main query or the query is happening in the WordPress admin
		if ( ! $wp_query->is_main_query() || is_admin() ) {
			return;
		}
		if ( is_tag() ) {
			// Get the taxonomy details of the post_tag taxonomy
			$taxonomy = get_taxonomy( 'post_tag' );
			// Sanity check so we don't throw any PHP errors
			if ( ! empty( $taxonomy->object_type ) ) {
				// Query for any post type that is associated with the post_tag taxonomy
				$wp_query->set( 'post_type', $taxonomy->object_type );
			}
		}
	}

}

// Kick things off
new CTF_Register_Taxonomies();

The original gist had if ($wp_query->get('tag')) $wp_query->set('post_type', 'any');

$wp_query->get('tag' would check if the query contains a slug of a tag and if so determines that yes this is a tag page. A better way would be to use [is_tag()](https://developer.wordpress.org/reference/functions/is_tag/) whose purpose is to determines whether the query is for an existing tag archive page.

We should also check to make sure the query is the main query and not a secondary loop on the page using $wp_query->is_main_query(). We also don't want to modify the behavior if the loop is being performed in the admin section of WordPress which we check with is_admin().

$wp_query->set('post_type', 'any'); would be fine for most situations. It's a good idea to be a little more specific and set the query to look in all of the post types that are associated with the post_tag taxonomy which is what is in $taxonomy->object_type.

We could go a step further and use $wp_query->get( 'post_type' ); to get all of the current post types being queried and then only adding new post types. Currently if there was another part of the theme or a plugin modifying the query our changes would overwrite their changes.