Experiment 1: When I was playing a Skyblock map, I found out that when falling, I begin getting void damage at Y~+25. Maybe even higher, +25 is when I noticed it.

Experiment 2: I enabled Education Edition commands for this map to unlock flying ability ("mayfly") and flew under the island, highest Y on which I received void damage this way is Y=+55. It's worth noting that this void damage is inconstant, unlike on negative y, and may work this second and not work next second even if you don't do anything.

Experiment 3: To ensure it's a vanilla bug and not a modded thing, I cleared 100x100 zone in a normal world filling it with air using commands and built a small platform above me. I enabled the mayfly ability again and changed gamemode to Survival, and I got void damage again.