And to be sure, I checked another domain with the same TLD to rule out the option that Quad9 is unable to handle the .pet TLD, but I received a correct answer...
Because that's their thing. That's the first thing that's right in your face when you open their website. It's like asking why AdGuard or even PiHole block DNS results.
Because it is a website created by a Kiwifarm transphobic individual who is scalping data from discord and selling access under the disguise of "seeing what your friends are up to" when in reality it is used to harass people.
Quad9 blocks lookups of malicious host names from an up-to-the-minute list of threats. (...) If the system detects that the site you want to reach is known to be infected, you’ll automatically be blocked from entry – keeping your data and computer safe.
I would never use a provider that has the means in place / likes to filter the DNS - you never know when a govt will ask them to kill a certain domain (even for the "unsecure" servers) or redirect things.
Don’t all providers have the ability to filter things?
It depends on what you can consider "the ability". If by ability you mean have to deploy a team of engineers working for a week to make it happens, that's okay, if they've their system built for it things are different.
Making a DNS server not respond to queries for a specific name is trivial for any DNS provider to implement, this is not a situation where they would have to develop months worth of new features to support that if the government asked.
Making a DNS server not respond to queries for a specific name is trivial for any DNS provider to implemen
It might not be that easy, you're thinking about one single server running some kind of DNS server you're familiar with. When we're talking about Quad9, Cloudflare etc. were talking about hundreds of servers across the planet, highly distributed solutions that rely on multicast and other non-trivial techniques. If you've to change a system like that to add the ability to block something, trust me, it won't take a few hours.