react-router - Maintain ReadonlyMap and ReadonlySet types in server response data. (#13092)
react-router - Fix basename usage without a leading slash in data routers (#11671)
react-router - Fix TypeError if you throw from patchRoutesOnNavigation when no partial matches exist (#14198)
react-router - Properly escape interpolated param values in generatePath() (#13530)
@react-router/dev - Fix potential memory leak in default entry.server (#14200)

⚠️ Unstable features are not recommended for production use

Client-side onError

react-router - Add <RouterProvider unstable_onError>/<HydratedRouter unstable_onError> prop for client side error reporting (#14162)

Middleware

react-router - Delay serialization of .data redirects to 202 responses until after middleware chain (#14205)
react-router - Update client middleware so it returns the dataStrategy results up the chain allowing for more advanced post-processing middleware (#14151, #14212)
react-router - Remove Data Mode future.unstable_middleware flag from createBrowserRouter (#14213)
- This is only needed as a Framework Mode flag because of the route modules and the getLoadContext type behavior change
- In Data Mode, it's an opt-in feature because it's just a new property on a route object, so there's no behavior changes that necessitate a flag