This skip was discovered by me. Steps to reproduce:

1. Jump up on the desk.
2. Jump up on the side of the shelf.
3. Face away from the wall.
4. Triple jump up, then backflip to get out of bounds.
5. Turn slightly right, and wait about 1 second.
6. Engage your wings.

If you did it right, you clipped down a level, to the 4th boss' area. There are several ways to fail this however, including falling out of the world and getting into a disconnect loop, going too high and clipping back to where you were, going slightly too high and clipping into an area where you're stuck, etc. Good luck!

Unfortunately the 4th boss isn't there though, so you gotta backtrack and defeat the 3rd boss to spawn it in. The 3rd boss' area also has a large invisible wall, which you can get over with a triple jump 180° backflip.

Overall, this only saves about 10 seconds unfortunately.

Items used:

• Wearing: Onyx Amulet with Wind Sculpting Stone & Harmonic Music Stone, Scroll of Momentum, speed enchants
• Before entering: Fried Bonefish, Charged Phial of Alacrity
• During run: Quickwick Candlestick, Soulshape, Nitro Boosts

Note that Azerite gear is no longer active in Shadowlands.