UPDATED: 04/22/2022

I was checking out some datapacks, saw some crawling ones, and to be frank, I just didn't like the way they were, so I made my own!
Big thanks to the head-smackers datapack.
I wouldn't have been inspired otherwise. Go check them out, leave some love. :)
https://www.planetminecraft.com/data-pack/head-smackers/

HOW TO USE:

1. Find a space to crawl in.
2. Approach the space.
3. Get up close and personal to the space.
4. Crouch while looking into the space. (DO NOT look directly down, because you MUST look INTO the space.)
   
5. Done

Your character should be prone now. :) Pretty simple. Have fun, and use this pack however you like. This is my first datapack after all.

KNOWN BUGS: All bugs are easily avoidable. Running into bugs may cause slight damage.

1. Trap Door - Crawling into a trap door is fine, but when the trap door is closed, the player WILL take damage when attempting to crawl in it.
   
2. Signs - The player is not able to crawl into a space occupied by a sign.
3. Steep Angles - May cause stuttering.

Big thanks to synncity for pointing out that cave_air was a thing. I totally didn't believe them at first and thought they were trolling me until I tested it.