systemd-resolved is a separate project from systemd whose only relation is that it’s in the same repo. It’s up to your distro builder to decide if they want to use it. And also up to your distro if they want to use the stub resolver. It’s not necessary for systemd-resolved to work. The reason they have the stub listener is because on Linux every application is supposed to use glibc nss to resolve names but there are real-world applications that try, incorrectly, to parse resolv.conf and do dns queries themselves. This breaks things if your assumption is that all dns queries are going through systemd-resolved. And this project isn’t the first. Ubuntu and others have used dnsmasq as the stub resolver for the same reason. Using just the dns nss module and resolv.conf is fundamentally broken and at this point unfixable when you are connected to multiple networks or with separate dns or split tunnel VPNing. You have to provide your own, smarter, nss module and/or use a stub resolver to cover these use-cases.