If you are standing on suspended scaffolding placed at Y=0, so you are standing at Y=1, you will take void damage and die.

To recreate, go to an open cave that is around Y=0. Tower up with scaffolding so that the top one is at Y=0 and climb to the top (you'll be standing at Y=1). Branch out horizontally a few scaffolds and walk out on the branch. With nothing below you, you'll take void damage and die. If there is anything below, even another scaffold, you are fine.

World download: 

https://drive.google.com/file/d/1eloSOcVHdOMqOBIh1NJ1TqIXORQrCiqO/view?usp=sharing