The bug
When you are exactly having a y-rotation (yaw) of -90 (exactly the positive X direction) you don't take kinetic damage when gliding with an elytra against a walI.
This can be seen in Xisuma's video ["Minecraft 1.16 Sixteen Nether Update Myths [Minecraft Myth Busting 128]"|https://youtu.be/a86-mlGmnu4?t=712] at 11:52.
Reproduction steps
Change your gamemode to Survival mode
Glide with an elytra towards a wall in the east
Run this command before you hit the wall: /tp @s ~ ~ ~ -90 ~
Make sure to not move your mouse
❌ You won’t take any damage, but when you move your mouse to change the direction, you will take damage.
Reproduction steps (complete setup)
Change your gamemode to Survival mode
/setblock ~2 ~ ~ repeating_command_block{auto:1b,Command:"teleport @p[nbt={FallFlying:1b},distance=..2] ~ ~10 ~ -90.0 20"}/fill ~18 ~ ~ ~18 ~3 ~ sandstoneEquip an elytra
Stand on the command block and start flying (don't move your mouse)
❌ You do not take any damage when colliding with the wall
Updated this report with the information from MC-194228. I hope this is alright.