Last updated: December 2, 2019
It seems like systemd-resolved made it to Pop!OS 19.10, making it the first distro I ever made /etc/resolv.conf immutable on after modification. systemd-resolved being the reason.. just.. why...
So I went and looked around as to what the purpose of systemd-resolved being a DNS stubby would be. And it appears to replace the DNS cache that already exists even in many home networks (as part of the router).. with a dedicated (sorta) DNS server running on 127.0.0.53. It's just a DNS cache that will query the "real" remote DNS server if it doesn't have a local cache, and will retrieve from its cache up to 4000 records (if memory serves me right) if it's already been cached there. Due to appearing as a "real" DNS server to client applications interacting with it, they will always query the stubby instead.
So with 2 potential caches I fail to see the point of systemd-resolved as yet another one. Especially since it introduces its own set of problems. Imagine chrooting into a distribution that uses systemd-resolved and (of course) pushes it into the /etc/resolv.conf. Now in your chroot systemd won't be pid1, so it will fail to run there.. "can't operate". And with it so will systemd-resolved. And where does that put the 127.0.0.53 in that chroot's /etc/resolv.conf? In an inherently dysfunctional state. Networking all gone.
And the best part of it? I do already have 2 local DNS servers to serve as local authorities for some domain extensions (.lan and .vpn) and to provide caching for everything else. They have a latency of around 0.160ms, caused by mostly wiring length. They're already more than sufficient as DNS caches, proper ones. systemd-resolved has no tangible benefits whatsoever when compared to those. But it doesn't need to be that fancy - if your router provides a DNS cache of its own you've got more or less the same thing.
Is there any reason for systemd-resolved to exist? Especially given the nonexistent "problems" it attempts to solve?