When you encounter d.adguard-dns.com, you are essentially hitting a signpost for a wider infrastructure. This is not a standalone website where you would find product information or customer service. Instead, it operates as a network endpoint. Because it redirects you to the parent domain, you should judge the safety of this link based on the reputation of the parent company, AdGuard.
Most technical infrastructure domains are designed to be invisible. The fact that this domain does not have deep search history or social media profiles is expected for this type of service. If you are checking d.adguard-dns.com to determine if you can safely use their software, focus your review on the main AdGuard website. That is where you will find the privacy policies and setup guides that govern your interaction with their tools. Is d.adguard-dns.com a scam? The evidence suggests this is simply a configuration for their DNS traffic. As long as the redirect lands you on the official adguard-dns.io portal, you are in the correct place to manage your network settings.