Unity Ads (or any other network) doesn’t show ATT popup. What you see there is the GDPR popup, that is mandatory to show for EU users.
But I understand that it should not be shown if you have previously shown the ATT popup and the user has denied it.
Take a look at this:
https://unityads.unity3d.com/help/resources/ios14-publisher-guide
Make sure you’re using Unity Ads 3.5.1 or higher.
What you could do is don’t initialize unity ads until you have shown the ATT popup, and wait until the user clicks on it.
After that you initialize Unity Ads, and unity ads should take into account the user choice. If they don’t have access to the IDFA they probably won’t show the GDPR popup.
But if they do (I’m not sure because I use admob mediation), what you have to do is pass the GDPR consent to Unity Ads before initializing, so they don’t show that popup ever. I do that in my games and mediation. I show my custom dialog and pass the consent to them.
You can see here how to do it (first bit of code)
https://unityads.unity3d.com/help/legal/data-privacy-and-consent
So if the user has denied the tracking, you just set the consent to false before initializing unity ads and that popup will not be shown.