it's only artificial if you created the game with diagonal whipping in mind then took it out to make it harder.  This did not happen, and quite honestly the game isn't even that hard so I don't understand the complaining at all.  As others have said diagonal whipping is not standard, it was only in 1 game out of many.

Fully granted, but even if it's not a necessary feature, it's still a very nice one to have. The example that comes to mind is Super Ghouls 'n Ghosts: the ability to redirect yourself after you've jumped was deliberately omitted, so it clearly wasn't designed with that feature in mind. Yet its inclusion would have helped to make the game much better than it was. Similarly, diagonal whipping in this game is probably  not necessary, but I'm certain it would have helped make the game better.* Increased player control is almost always a better way to do things, in my opinion.

*Obviously, I can't say for sure, since I haven't played the game yet.

 It's not about whipping diagonally, it's about being able to attack diagonally (hence my "if you equipped one item or another" qualifier). Correct me if I'm wrong, but I believe in Rondo Maria had a diagonal attack, and all but one of the Metroidvanias has an equipable item that can do such an attack (Circle of the Moon is the exception).

That said, I do have to backtrack a bit now, since this game probably does feature a viable diagonal attack (the Axe).

 I think I've covered this in the earlier segments of this post, but I will add that I don't think that's the only way to create artificial difficulty. It's more of a philosophical discussion than anything, though.

Well if all you need is the axe.... the question becomes this: can you get the AXE in this game?

If you're an old school Castlevania, you've GOT to have the Axe!

ghouls and ghosts is a good example because they had it so you could attack upward and downward in the genesis version, then they took it out later. They added the double jump but they got rid of the upward and downward attack and it made it a better game. They got rid of a bunch of the frustrating castlevania things in rebirth, but they didn't put diagonal whipping back in.

well there is an axe in the game of course, one of the more rare ones. If you don't die and dont' take other items you can keep it though.

Yes.  And also the dagger, stopwatch, holy water and (boomerang) cross.

how is the difficulty compared to say a game like Mega Man 9?

Metroidvania. XD
OT: I've already beat the GBA version. I don't think I wanna try this.

pretty easy in comparison, rebirth is not that hard of a game depending on what settings you put it on.