At the bottom of the Adguard DNS page they have FAQs, and one of them:
Why is AdGuard DNS free?
You can use public AdGuard DNS servers for free, there's no catch. However, we incorporate AdGuard DNS in many of AdGuard and AdGuard VPN paid apps. We are also going to offer paid personal AdGuard DNS servers with advanced functionality and more customization options.
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At the bottom of the Adguard DNS page they have FAQs, and one of them:
Why is AdGuard DNS free?
You can use public AdGuard DNS servers for free, there's no catch. However, we incorporate AdGuard DNS in many of AdGuard and AdGuard VPN paid apps. We are also going to offer paid personal AdGuard DNS servers with advanced functionality and more customization options.