Six pages of posts and not a single poster can produce the rule? As far as I can find:
WA Technical Rule 22.6 (p. 40): "In addition, an athlete shall be disqualified, if:
22.6.1. their foot or leg is, at the instant of clearance, beside the hurdle (on either side) below the horizontal plane of the top of any hurdle;"
That being said, from the videos and stills, it looks like Warholm's left lead leg is slightly below the horizontal plane of the top of the hurdle as he brings it up. His foot then continues upward for some distance above the horizontal plane. In other words, the lead leg "appears" to clear more than 36 inches (if not at the hurdle, then immediately thereafter), rather than a case of his lead leg remaining below the horizontal plane. This would seem to be the intent of the rule to ensure that the hurdle height is actually cleared. So, no advantage gained, IMHO.
Also, having your left lead leg slide around below the hurdle would tend to cause the lead leg to land on the inside lane line in addition to the likely outcome of having your trailing leg slam into the hurdle on the way over. (And for a left lead leg High Hurdler, this situation could only occur running in lane one).