Use term ids for tag links

term-link-id.php

<?php

/**
 * Transform 'tag=' (which expects slug) to 'tag_id=' in rewrite rules.
 */
add_filter( 'post_tag_rewrite_rules', function( $rewrite_rules ) {
	$processed_rules = array();
	foreach ( $rewrite_rules as $match => $query ) {
		$processed_rules[ $match ] = str_replace( 'tag=', 'tag_id=', $query );
	}
	return $processed_rules;
});

/**
 * 'tag_id' is normally a private query variable, and we need to make it public.
 */
add_filter( 'query_vars', function( $query_vars ){
	$query_vars[] = 'tag_id';
	return $query_vars;
});

/**
 * Transform tag links to use term id instead of slug.
 */
add_filter( 'term_link', function( $link, $term, $taxonomy ) {
	if ( 'post_tag' !== $taxonomy ) {
		return $link;
	}
	return home_url( 'tag/' . $term->term_id . '/' );
}, 10, 3 );