Hi everyone.
This endpoint was never a part of our documented API, and was in fact recently removed as part of some internal cleanup.
Of course we recognise that a number of you were using this method of redirect in your apps, and this leads to an unfortunate situation where you’re now finding that the calls no longer work. For those of you who did previously make use of this redirect, we suggest that the follow user intent part of Twitter for Websites is a supported alternative way of getting to a user profile based on user_id.