apps/docs/src/pages/docs/[...slug].tsx

@@ -58,7 +58,8 @@ export default function DocPage({
   const [active, setActive] = useState("");
   
   const siblings = docs.filter((d) => d.collection === doc.collection);
-  const collectionEntry = collectionEntries(docs).find((c) => c.name === doc.collection)!.entry;
+  // Historical pages remain readable even when their collection is outside the catalog.
+  const collectionEntry = collectionEntries(docs).find((c) => c.name === doc.collection)?.entry ?? doc;
   const index = siblings.findIndex((d) => d.slug === doc.slug);
   useEffect(() => {
     const observer = new IntersectionObserver(