I've been wondering about this and doing a lot of thinking and watching footage- and I'd have to say that my experience and research (while not definitive) would point to the tipman spike not existing in PM
as it did in Melee.
I would point to how in almost every PM video with Ganon in it (that I've seen), even the best Ganons almost never attempt it, and when they do, it doesn't work. This alone would suggest that if it's still possible, it's no longer viable compared to edgegaurding with a bair or what have you.
I wanted to make sure that it wasn't the hitboxes- because I, too have suspected this for some time. So, I found what data I could on the Melee and Brawl hitbox data (none available for PM), took screenshots of the last frame of both hitboxes, and laid one over the other:
(The overlay is Melee Ganon, the underside is Brawl... in case you couldn't tell)
Obviously, I used the size of the actual hitboxes as a scaling reference. It looks like the rest of Ganon doesn't scale properly, but the back leg and torso actually match up quite nicely. I'm too lazy to post a picture of that, though.
This suggests, quite strangely, that if Ganon's uair hitbox goes further
down (in relation to the entire Ganon model), then the tipman spike should actually be
easier... but it's not.
What I'm thinking now is... maybe the uair hitbox doesn't actually go through the stage (and under the ledge?) I know I've made a lot of assumptions in this thread, like the idea that we can just lay a Melee hitbox over a Brawl hitbox and it'll tell us something about Project M,but I absolutely don't have the time or the ability to explore this question in depth at the moment.
TL:DR, you guys figure it out.