I made a post a few days ago where I listed some rules I suggested for what counts as a skip. That post has been deleted because, as it turns out, those rules were crap. The following is a list of every shortcut that I'm aware of and my suggestion for its ban status:
Warm Up: Jumping down from start (banned) U-turn skip (banned)
Beginner: Skipping last ramp before each bridge
Intermediate: Double corner cut Small ramp skip after tube #2 Trap jump (banned)
Dizzy: Tube #1 skip (banned) Alternate spiral entry (banned) Spiral skip (banned) Water wheel skip (banned)
Tower: Path swap (banned) (useless)
Up: Tube #2 skip (banned) Loop skip (banned) Boost ramp detachment (banned)
Neon: Light peg skip (banned) Spiral #1 skip (banned)
Expert: Big ramp to goal (very hard)
Odd: U-turn #1 skip (banned) U-turn #2 skip (banned) Left-gravity #1 skip (banned) Left-gravity #1 to left-gravity #2 (banned) 8-ball #1 to twin spiral (banned) Left-gravity #2 to goal (banned) Right-gravity #2 skip (banned)
Toob: Tube #1 skip (banned) Upper path skip (banned) Tube #6 skip (banned)
Wobbly: Steep ramp skip (banned)
Glass: Bridge to nowhere (ridiculously hard) (banned)
Sky: Pre-glass skip (banned)
Master: 1st cycle bridge (banned) Bridge drop (banned) Hammer skip (banned) Tube #4 skip (banned) Catapult skip (banned) Tube #5 skip (banned)
Impossible: Catwalk #1 skip (banned) Catwalk #3 skip (banned) Tube #2 skip (banned) Catwalk #4 skip (banned)