Is it illegal to rack the slide of a pistol in order to issue a threat?
Miyamoto Musashi, the legendary swordsman, said something that you might find beneficial. It goes something like this:

“If a man insults you and you feel he must die for his insult, draw your sword and cut him down. If you simply want to humble him but not kill him, keep both your tongue and your sword sheathed for only a fool communicates his intent before he acts.”

Now, to your question:

In most localities, I believe it is illegal. It’s also stupid. If you are carrying, you should have a round in the chamber because violence happens quickly and adding extra movements- especially with the adrenaline dump that often happens when shit goes down- could get you killed. Remember that, in most cases, people experience that dreaded adrenaline dump and lose their coordination among other things.

Do not brandish it to threaten- that’s called “menacing” in many locations. If you are going to draw your firearm, do so because you couldn’t leave safely after the other person showed his/her intent, means, and had the opportunity to do it. At this point, you are drawing to stop a threat from which you cannot safely escape. If the aggressor then moves forward, drop him like a bad habit. It is generally considered self-defense.

If you do, you must be able to clearly articulate “why” you did it, though, or you’re in for a lot of trouble.

Just for fun, let’s say you “rack your slide” to cycle a round and the object of your intimidation tactic is carrying concealed. While you’re showing off, he shoots you and you have a new anus in the middle of your forehead. That’s a terrible reason to die, don’t you think? Getting your head ventilated because you were showing off to intimidate?

On a more calm note, though, I’ve found that when I’ve had to draw (I’m one of those concealed carry folks), the weapon itself is usually enough to change someone’s mind- predators like easy targets and have no desire to get hurt for a number of reasons including that being injured then makes them prey to other predators. I had that experience with a young man not too long ago who apparently thought my Cocker Spaniel and me were an easy target.