To understand this situation you must first start by looking up the definition of Known or Virtually Certain in issue of the Rules (players edition, Rules of Golf, or Official Guide). Then go to the Official Guide and look up Interpretation # 16.3a(2)/1. Also read rule 16.1e (ball not found in Abnormal Course Condition) paying close attention to the “But” at the end. See how simple the USGA has made this, haha. Basically if the ball can be anywhere else than the ACC or Penalty area, it is LOST. Someone has to SEE exactly where the ball crossed or entered . So over the hill might be a problem, or did you hear it hit a tree or limb which could have altered the flight, etc, or was the slope of land such that the ball had to roll out ant into penalty area. In other words, it is now a clear cut answer to the question and is probably on a case by caee and seeing exactly all the surrounding area of where it happened.
As for the Slope edition, I do not think, and I do play with a Slope Edition, gains anyone as much of an advantage, because you still have to hit the shot and most people I play with, after determining what the distance is changed to, still do not account for the fact the ball may be on a downslope or an upslope and do not enter that into the calculation. I do think that the people that do not interpret the Temporary Water correctly, or Nearest Point of Relief (not Nicest) or where it entered penalty area gain more advantage. Over the years I have been shocked at the total lack of knowledge that the majority of players that play in the WA have on the rules. The first WA I played in (2005) I went to a seminar the WA had and it helped me tremendously. They currently do some type of WEB thing which I have not checked on but have heard other talk about it.