You can't compare running a single service in your home network with the operations it takes to serve a public DNS
This is a scaling problem, not an intrinsic difficulty. Mullvad already had the infrastructure in place. I suspect the real reason is cost-cutting.
>https://freedns.afraid.org/
This dude has been doing it for 25 years, and actually this is a dns provider for domain names which I'm decently sure makes it more complicated than public dns.
They're allowed to say they don't want to pay for it anymore, I just think their logic is bad. Or maybe their lawyer said they're running risks just ignoring takedown and they didn't want to deal with it anymore.