I has a crawling glitch with a slab but i found a glitch without slabs.
Example :
1.Use Elytra and Trident (Riptide III Echantments)
2.Hold Right click
3.Just jump fast (to fly the elytra), and Facing down
4.Stop the right click and go the under block (not slabs)

If you don't know see my this image.