Booking.com does not do it. Once an A/B test shows positive outcome with a high enough confidence level, the experiment goes "full on" and is shown to everybody.
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.