Issue with custom slug validation in Sanity Studio v3 is resolved.

import { useClient } from "sanity";

export function isPageSlugUnique(slug, options) {
  const client = useClient({ apiVersion: "2022-02-03" });
  const [result, setResult] = useState(false);

  const { document } = options;
  const id = document._id.replace(/^drafts\./, "");

  useEffect(() => {
    async function fetchDocument() {
      const params = {
        draft: `drafts.${id}`,
        published: id,
        site: document.site,
        slug,
      };

      const query = `!defined(*[!(_id in [$draft, $published]) && slug.current == $slug && site == $site][0]._id )`;
      const res = await client.fetch(query, params);

      setResult(res);
    }
    fetchDocument();
  }, []);

  return result;
}