It should be there. I do it most games. But it won't show if their number or proximity aren't threatening. I have no idea what the formula is, but since they're AI, you can probably feel secure when you can't see the option.
Minimum of 3 visible combat units within 4 tiles of one of your cities, units within the AI's own borders count as half. Can't ask if you've given them Open Borders or you have a Declaration of Friendship/Defensive Pact.
I assume this is the same for the AI as well? I always thought it was slightly different but close enough that I could avoid it 99% of the time.
Note that making the request to the AI has a high chance of triggering war immediately, when they may have left you alone otherwise. I recall a game I had when I asked Isabella to remove her troops, and she immediately declared war. I reverted my save and just ignored her troops the second time around, and the war declaration never came. Though that was a few patches ago, and the AI behavior may have changed since then.
I wouldn't say "high" chance, because I've had it go the other way often enough. The odds probably do go up a bit, if the AI is insulted by the request. Maybe it's more accurate to say just say "a chance." It's still something to consider when you weigh confronting the AI. Basically it comes down to "are you okay with a war"?
The one positive thing is if they declare war you get to attack first, & can damage a few troops, particularly if you fear an attack & have significant troops in the area.
I currently have a situation where a sanctioned Zulu is guarded after losing a war to me. He's recompiled his army/navy in anticipation of more war, and he's promised not to attack after I asked him to move his troops. My question is, do I get any kind of negative diplo (with him or other civs) if I attack him within these 20 turns? I would assume no, considering I didn't promise him anything.You also don't get the warmongering penalty.
I currently have a situation where a sanctioned Zulu is guarded after losing a war to me. He's recompiled his army/navy in anticipation of more war, and he's promised not to attack after I asked him to move his troops. My question is, do I get any kind of negative diplo (with him or other civs) if I attack him within these 20 turns? I would assume no, considering I didn't promise him anything.