Your tag archive hasn’t stopped working because it never existed to begin with. WordPress displays items via the main loop and this takes 2 forms:
- Individual Posts
- Post archives
If you want to list the categories or tags of your site, you’ll need to create a page for that and use code be it via a plugin and shortcode, or a custom template in your theme
Such a theme template could contain a redirect by using wp_safe_redirect( home_url() ); before outputting any markup