Flying with an elytra and colliding with floating flowing water sometimes doesn't reset fall damage.
- Create a creative world with a large enough vertical hole.
- Place some signs a few blocks above the bottom of the hole, as shown in the attached video.
- Place water above the signs.
- Switch to survival mode.
- Give yourself elytra and fireworks and fly down the hole at high speeds.
Eventually you will die of fall damage.
Water would reset the fall damage.