set-post_tag-taxonomy-non-public.php

<?php
/**
 * Overwrite default post_tag taxonomy to be non-public.
 * 
 * @see https://developer.wordpress.org/reference/functions/register_taxonomy/
 * 
 * @return void
 */
function vip_modify_post_tag_taxonomy() {
	$post_tag_args = get_taxonomy( 'post_tag' );

	$post_tag_args->public = false;
	$post_tag_args->publicly_queryable = false;

	register_taxonomy( 'post_tag', 'post', $post_tag_args );
}

add_action( 'init', 'vip_modify_post_tag_taxonomy', 20 );