[WordPress] タグアーカイブのパーマリンクをタグIDにする ref: https://qiita.com/gatespace/items/4548fd0b9540f46b86b1

file0.php

<?php
function numeric_term_link( $url, $term, $taxonomy ) {
    global $wp_rewrite;

    // post_tag only
    if ( $taxonomy == 'post_tag' ) {
        if ( $wp_rewrite->using_permalinks() ) {
            $permastruct = $wp_rewrite->get_extra_permastruct( 'post_tag' );
            $permastruct = str_replace( '%post_tag%', $term->term_id, $permastruct );
            $url = home_url( user_trailingslashit( $permastruct, 'tag' ) );
        }
    }
    return $url;
}
add_filter( 'term_link', 'numeric_term_link', 10, 3 );

function add_tag_id_to_qvar( $vars ) {
    $vars[] = 'tag_id';
    return $vars;
}
add_filter( 'query_vars', 'add_tag_id_to_qvar' );

function numeric_tag_rewrite_rules( $rules ) {
    $custom_rules = array();
    foreach ( $rules as $regex => $rewrite ) {
        $regex = str_replace( '/(.+?)/', '/([0-9]{1,})/', $regex );
        $rewrite = str_replace( '?tag=', '?tag_id=', $rewrite );
        $custom_rules[$regex] = $rewrite;
    }
    return $custom_rules;
}
add_filter( 'post_tag_rewrite_rules', 'numeric_tag_rewrite_rules' );