I created a repeating command block with the following text:

/kill @a[y=-100]

In order to skip void damage. This killed all players at all y levels.

I ran a test to check if it was my command or something else.

I placed another repeating command block with this text:

/execute at @a if @p[y=-55]

And I placed a comparator next to it.

The comparator was continually lit no matter what y level I went to. I was the only player in that world.