You can clip through walls using shulker boxes

You can clip through 1 block walls using shulker boxes

How to recreate:
1 - Place a shulker box with the top facing the wall you want to clip through.
2 - Stand close to the shulker box and open it.
3 - Close the shulker box and move backwards to phase through the wall.

Shulker boxes slightly push you into the wall, allowing phasing.

If you stand close to the wall instead, it will quickly push you out of the clipping area