I know this was an old ruling but i cant seem to find where this would be in the current rulebook.
If you lose flight/incorp/ghostly or anything else that allows you to move over an obstuction or other model while overlaping said model or obstruction what happens?
It used to be you just reverted to your last legal position. Just want to be sure thats still the case.
I’ll answer Flight right now, but the same basic argument applies for Incorporeal. (I don’t have the text of Ghostly at hand and I’m too lazy to look it up. 
)
Here’s the key sentence about Flight:
A model without Flight loses the highlighted sentence, and thus (barring other special rules, etc. and so forth) cannot advance through other models. If the model with Flight is overlapping another model’s base when it loses Flight, then it loses the ability to advance through other models.
As the old saying goes, the rules don’t tell you what to do if you’re not following the rules. 
In this situation, you’d have to rewind that movement because it’s not a valid movement. (And the fact that the movement only becomes invalid in between doesn’t matter. We can take a cue from Rough Terrain and understand that rewinding is the only logical choice.)
I had posed this question on the discord, it appears you answered a question that wasn’t being asked.
The rule specifically in question is here;
The model with flight, say Israfyl with Arcane Propulsion from Scyrafael, advances onto the obstruction. The model then has the effect dispelled, either dropping upkeep or an enemy attack/spell/effect. The model validly advanced onto the obstruction but now has lost flight.
The text you present adds to the issue. Is the model essentially stuck in place on top of the obstruction as it has no way to advance “through” the obstruction? Or is it “on” the obstruction and ignores these rules? What separates “through” and “on” in the rules? Etc.
I believe I answered the question Wolfe asked.
If the loss of Flight/Incorporeal/Ghostly happens in the middle of the movement while the moving model is in an invalid position, then you have to rewind the movement, because you’re attempting to make an invalid movement.
If the models lose the rule after the movement is complete, there is no further interaction. Whether it’s been 8 seconds or 8 turns, the whole advance was resolved, and there’s no reason to go back.
In your example, if the model had Flight and ended its movement on the obstruction, it has made a legal movement. If it loses Flight during the opponent’s turn, nothing happens, because no rule (either a standard rule or a special rule that I can think of) tells us that anything happens.
If the model is on top of the obstruction and wants to advance off the obstruction, then this sentence is the only part that I can see that would apply:
“Obstructions are difficult terrain.”
So, unless the model has Pathfinder or an equivalent, you’d reduce the distance the model moves by 2" while moving off the obstruction (or “climbing down” or whatever you want to call it), and that is that.
Models with Flight can move through obstructions, but “through” is not “off”.
