This is why you have long-term holdbacks: a group of users who never sees a set of experiments. That way you can measure over a longer period of time the true impact of a set of experiments.
Long term holdbacks actually are way way harder to implement and maintain in a fast moving product than meets the eye. We had passionate and ultimately unfruitful conversations about how to do that when I was there in 2016.
The only thing we had at the time was continued tracking against the experiment groups after flipping that switch.