Did Q ever refer to it as such? It's been part of the general lexicon for so long that I can't even remember it's origin.
When was it first used? Why is it referred to as a killbox?
In military language a killbox is a 3D target area defined to facilitate the integration of joint weapons fire.
Was there anything that Q said that marked these bracket usages as such? Or was that an anon idea that took off?
True, the phrase "kill box" does occur in #680, but I think OP is asking about the use of brackets [ ] to indicate a person or agency is in said "kill box."
Q is referring to the same thing. Visit the Q post directly. The screenshot is not complete
https://qalerts.pub/?n=680
Aha! There it is...thanks.