players can start crawling under shulkers one block off the ground at specific coordinates

When a player collides with a shulker one block off the ground, such that the face of the shulker the player is colliding with is precisely some power of 2, it is possible for the player to automatically enter crawling mode. This works for the following powers of 2:
+-2, +-8, +-64 and probably even more which I haven’t tested.