According to conventional adtech and martech, exactly 0% of users are blocking tracking by conventional adtech and martech.
You can look at ad blocking rates by market to get an idea of how many "invisible" users there are, but the "headline rate" for ad blocking has two problems.
The ad blocking rate mixes up users of three kinds of software: tracking protection tools that block only third-party tracking (such as Privacy Badger), pure ad blockers that block both third-party trackers and ads (such as uBlock Origin) and ad blockers that run paid whitelisting schemes (such as Adblock Plus).
Ad blocking and tracking protection percentages are vastly different from site to site. Some sites serve a community of practice whose members install a lot of privacy tools. Some sites don't.
For example, several sites that cover web development and devops are getting tracking protection rates around 30%. One site is around 40%. But if you look at the average tracking protection rate for a country, you don't see how much of a brand's audience is less trackable. (You can't walk across a river that is an average of two feet deep.) What does this mean for brands?
If you're not reaching people, you're not reaching them. Most of today's web advertising is invisible to users of some privacy tools and ad blockers.
Higher blocking rates don't necessarily mean you get billed less for ad impressions. They can mean that more of the impressions you do get billed for are bots.
Data-driven marketing decisions get you the wrong answers if real customers get consistently missed.
What can brands do? The first step is to figure out if a brand has the kind of protected customers who make it necessary to do something. If all the customers are neatly trackable, that brand probably has higher priorities. (protip: brand safety)
If you run the test and the customers are better protected from tracking than the average, that's where the opportunity comes in.
While the competition wastes their budget on reaching the wrong people, or bots, they're not getting to the tracking-protected audience. Knowing if you have tracking-protected customers is the first step in building creative ways to reach them.