AdGuard provides several legitimate ways to get a key without paying: Beta Testing Program Apply to be a Beta Tester

AdGuard maintains a server-side blocklist of leaked and stolen licence keys. If you find a key online, it is likely already:

While there is no literal academic "paper" titled exactly after that search term, the phrase often appears on grey-market or tutorial sites offering "verified" keys. If you are looking for a way to get legitimately for free or at a discount, there are several official paths provided by the developer. Official Ways to Get Free AdGuard Licenses

This combination gives you 95% of the protection of a premium AdGuard license without spending a dime or risking malware.