Though never officially documented, there are tonloads of evidence that it does exist.
If you ever have 39 packs with no legendary from the same expansion, then the 40th is guaranteed to have one.
If you ever have 9 packs with no epic from the same expansion, then the 10th is guaranteed to have one.
Since this is not officially documented, you might not get any help if you believe that this went wrong for you and open a ticket. But based on evidence, this always works. (And the one time something did go wrong with the pity timer, Blizzard corrected stuff without officially acknowledging it was related to the pity timer, just saying that there was a bug that “affected legendary drop rates” or similar words).