Hmm... I was pretty sure there's a blockade early on in the desert that you can't pass without being able to do the wing flaps. Maybe I've messed it up... you're referring to the desert west of the truck, right?
It's pretty unlikely I'll ever fix things like this since Game Maker 8 is discontinued and doesn't run properly in Windows 10, and the engine uses functionality deprecated in Game Maker Studio, but thanks for pointing them out anyway! I'll make sure to have a look on them if I ever get around to making a 'finished' version.
The fake walls and stuff are in other places as well, anyway. I don't remember everything by heart anymore since I haven't touched the game actively in like 2 years, but from the top of my head, written in riddle form to reduce spoilers:
- The grass is green, but not all green is grass, might be something new where you've already been.
- The longest road is mostly flat, but there's something above your hat.
- An arrow of barrels, hard to see, still guides you to a mystery.
- Near the canal, your feet may burn; still there's a reason to not instantly return.
- In the sky, all's not what it looks; moreso than usual, think outside the box.
There's some other places I can remember, but I don't remember WHERE they are, so I can't really offer hints for those yet :P