Forum archive

Short answer: enabling a forum archive is fine — but only if you treat one URL form as the canonical one so search engines do not index the same thread under multiple addresses. The real problem is multiple accessible URL variants (archive pages, dynamic query URLs, session IDs, sort/page params). Pick one canonical address for each thread and make every other variant point to it.

Practical steps to prevent duplicate-indexing:

1. Choose the canonical URL format and use it for all internal links. Consistency is the easiest win.

2. Implement server-side 301 redirects from non-canonical variants to the canonical URL so crawlers receive a single permanent location. Example (adapt to your patterns):

   # redirect dynamic thread.php?t=123 to /threads/123-title.html
   RewriteCond %{QUERY_STRING} (^|&)t=([0-9]+)(&|$)
   RewriteRule ^thread\.php$ /threads/%2-title.html? [R=301,L]

3. Add a rel="canonical" tag in the page head that points to the canonical .html URL as a safety net:

   <link rel="canonical" href="https://www.example.com/threads/123-title.html" />

4. If archive pages are meant only for internal use and should never appear in search, use a meta robots noindex,follow on those archive views — do not rely on robots.txt to block them (blocking prevents crawlers from seeing meta tags and canonical links).

Verify and monitor: use Google Search Console to inspect which URL Google chooses as canonical and check server logs to see what variants Googlebot requests. For general guidance on consolidating duplicate URLs and canonical tags, see Google’s documentation and a practical explainer from Moz: Consolidate duplicate URLs and Duplicate content — Moz.

If @Dani enables the archive, follow the steps above so the archive either redirects to or explicitly points (via canonical/noindex as appropriate) to the single URL you want indexed.