docs: add section on deprecation of multipart form data and file uplo…

…ad utilities (#10338)

docs/start/future-flags.md

@@ -697,6 +697,19 @@ type SerializeFrom<T> = ReturnType<typeof useLoaderData<T>>;
 
 In most cases, you should be able to just remove `SerializeFrom` and use the types returned from `useLoaderData`/`useActionData`, or the types of the data in `loader`/`action` functions.
 
+### Multipart Form Data and File Upload utilities
+
+The following utilities are deprecated and will be removed in React Router v7:
+
+- `unstable_parseMultipartFormData`
+- `unstable_composeUploadHandlers`
+- `unstable_createFileUploadHandler`
+- `unstable_createMemoryUploadHandler`
+
+We recommend using [`@mjackson/form-data-parser`][form-data-parser] and [`@mjackson/file-storage`][file-storage] to handle multipart form data and file uploads.
+
+You can also checkout the [React Router "File Uploads" doc][react-router-file-uploads] or ["File uploads with Remix"][file-uploads-with-remix] blog post for guides on using these libraries.
+
 [development-strategy]: ../guides/api-development-strategy
 [fetcherpersist-rfc]: https://github.com/remix-run/remix/discussions/7698
 [relativesplatpath-changelog]: https://github.com/remix-run/remix/blob/main/CHANGELOG.md#futurev3_relativesplatpath
@@ -724,3 +737,7 @@ In most cases, you should be able to just remove `SerializeFrom` and use the typ
 [data-api]: ../utils/data
 [response-json]: https://developer.mozilla.org/en-US/docs/Web/API/Response/json
 [remix-template-eslint-config]: https://github.com/remix-run/remix/blob/main/templates/remix/.eslintrc.cjs
+[form-data-parser]: https://github.com/mjackson/remix-the-web/tree/main/packages/form-data-parser
+[file-storage]: https://github.com/mjackson/remix-the-web/tree/main/packages/file-storage
+[file-uploads-with-remix]: https://programmingarehard.com/2024/09/06/remix-file-uploads-updated.html/
+[react-router-file-uploads]: https://reactrouter.com/how-to/file-uploads