The most blatant piece of evidence for what they do is how they say "ONLY ONE ROOM LEFT AT THIS HOTEL" but when you go back and request two rooms, this changes to "ONLY TWO ROOMS LEFT AT THIS HOTEL" immediately.
During my time there, the company would never actually do something that was technically wrong (or lying), with a couple of exceptions where some individual went too far and wasn't caught in any review. There were certainly UI elements that were misleading or suggesting (never claiming) of something being a freebie when it was included in the chosen rate. These sort of changes consistently received internal criticism, but ultimately due to the outstanding A/B testing tooling, were shown to be worth too much money for the detractors to succeed. There were maybe a handful that were rolled back due to internal criticism.
In any case, it seems unlikely that booking would implement the change you describe.
NB: the actual availability was always controlled by the hotels, as were the prices - booking does not own inventory, unlike some of its competitors!
Edit: removed surplus punctuation.