I'm on Debian stable and there's no systemd-resolved or NetworkManager running. When I'm on a DHCP network dhclient runs and manages my resolv.conf as expected

I'm on Debian stable and there's no systemd-resolved or NetworkManager running. When I'm on a DHCP network dhclient runs and manages my resolv.conf as expected. We're still entirely in control in GNU/Linux, if you assert control. It turns out however that most people actually want the distros to do all the magic and for things to Just Work.