If you have a 3 block tall space with the bottom block being a full block you can put a trapdoor on it. if you open the trapdoor so you can crouch under the slab and then close it again, you go back to full walking speed under the slab and your head visually goes into the slab
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However, we are already tracking this issue, and this report is a Duplicate of MC-165423, which is a report that has not been resolved yet.
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