Preserve Query Strings on Old-Slug Redirects, If Any

[mkormendy]

WordPress's wp_old_slug_redirect() (wp-includes/query.php) builds its redirect destination via get_permalink() and drops the original request's query string entirely. This is unexpected, and the option to have query parameters be preserved to the new destination should be available.

A valid usage case is with advertising campaigns that link to old urls containing URL parameters for tracking. Advertisements may exist in permanent form (on paper) with a relevant code in the query string of the URL. If someone modifies the URL in WordPress and wp_old_slug_redirect() handles the redirect and points to the updated slug, then those query parameters should be passed along.

[mkormendy]

A workaround (in mu-plugin).

[westonruter]

This makes sense to me.

I modified the slug of the Hello World post from hello-world to hola-mundo. Then I renamed it back.

When I tried going to /2025/12/25/hola-mundo/?utm=foo I was redirected to /2025/12/25/hello-world/ with the query parameter removed.

The wp_old_slug_redirect() function was introduced in WP 2.1. I'm surprised this hasn't been raised as an issue before.

[mkormendy]

At my company, since implementing my bandaid mu-plugin fix, we are seeing a huge ads attribution bump with our traffic that was tied to businesses that closed during covid and reopened nearby recently.

## Summary

Preserves the original query string when wp_old_slug_redirect() redirects an old slug to the current permalink.

This keeps tracking params and raw query args intact during the redirect.

## Testing

Run:

`bash
npm run test:php -- --filter Tests_Rewrite_OldSlugRedirect