I'm attempting to remove certain animals from my world spawn. I had assumed that if I set the "negate" values to True in all the categories for that mob it would cause them to not spawn anywhere, but not so. Example emu_spawns.json:

emu_spawns.json.txt

Still spawning emus.

This is using version 1.9.1, but I haven't seen anything in the changelogs for later versions to indicate that this has been fixed.

Am I incorrect in my assumption that setting Negate to True cancels that spawn ? Or is this a bug ?

thats not how it works at all. change the emu spawn weight to 0 for this.