Edit: This was a fix to an unintended method rather than a nerf.
I didn't see this in the release notes, so I thought I'd share.
Pre-u9, if you cast a blade barrier, then jumped and cast another blade barrier above it without overlapping, mobs would take two ticks of damage each time they passed through the barriers.
Post-u9, they appear to still take one tick of damage when blades are stacked:
At first I thought it might be a fluke with the zombies since they are so short, but they still take damage from a single barrier that is high in the air: