Can we send a user to mobile web from an ad and then pass through campaign data into the app via deep link?

While this may be technically possible, our understanding is that it would fall under the definition of 'ad tracking' from Apple's policy, and is therefore subject to ATT opt-in.

In other words, there is no restriction on sending a user to mobile web and capturing conversion data there, but attempting to pass that ad campaign conversion data through to the app without securing user opt in via ATT could put your app in violation of your Apple Developer Program License Agreement.