Dumb question: why is this a thing at all if the kernel oom killer already exists? Doesn't this effectively do the same as the existing oom killer, but worse?
systemd-oomd (and earlyoom, which Fedora used for a couple of releases before switching) try to detect that you're running out of RAM before the system becomes unusable, and kill a big memory user. It periodically killed Firefox before I bought some more RAM, which was a pain, but less of a pain than hard-rebooting the laptop.
The plans for Fedora to do this are public pages:
https://fedoraproject.org/wiki/Changes/EnableEarlyoom https://fedoraproject.org/wiki/Changes/EnableSystemdOomd
The most facepalm one was somebody uploaded their `.vimrc` which had a bug that infinitely recursed the parent directory loading ctags until it gobbled up all available memory and crashed. So every time he opened vim, it started about a 40 second timer of doom unless vim was exited before memory exhausted. Took forever to figure it out too, because he often only opened files for 10 seconds or less, short enough not to trigger the OOM.