In iOS 10, when you enable “Limit Ad Tracking”, [the Identifier for Advertisers] now returns a string of zeroes. So for the estimated 15-20% of people who enable this feature, they will all have the same IDFA instead of unique ones. This makes the IDFA pretty much useless when “Limit Ad Tracking” is on, which is a bonus, as this is what users will expect when they enable the feature. These users will still be served ads, but its more likely they will not be targeted to them based on their behaviour.
Of course, there are lots of other ways nefarious ad tech companies can try to build tracking profiles, from device profiling to requiring a user account. This is a step in the right direction, though.