Sitewide advertising industry opt-out
The ads.txt specification provides a way to list the authorized sellers of ads for a domain. The app-ads.txt specification extends this to cover apps tied to the domain. As a domain owner, this is a valuable way to crack down on fraudulent usage of your domain including by adware.
For domains without any third party advertising including those without any ads at all, you should serve both
/app-ads.txt from a web server with the placeholder record defined by the specification:
placeholder.example.com, placeholder, DIRECT, placeholder
The placeholder record formally disallows buying and selling ads on behalf of the domain including for any subdomains. This prevents fradulently buying / selling ads for your domain anywhere that ads.txt / app-ads.txt are enforced.
It's in the interest of most ad tech companies to enforce these standards due to losses from ad fraud so adoption is increasingly widespread.
Browser extension malware injecting ads into sites is very common and this is a way for sites to hurt those malware developers where it hurts: their pocketbook.
These standards have a limited scope and were primarily created to address the cost of ad fraud for the advertising industry, but they do offer value for domain owners to protect their reputation and discourage adware.