the well-publicized incident to which you're referring had systemd introduce something that broke userland, then instead of fixing it in systemd code, proposed a kernel change for it. this is, obviously, bad practice.
it would be one thing if the systemd code exposed a vulnerability or inefficiency in the linux kernel. but fixing your bad code by modifying the kernel is like trying to modify a popular compiler because some bad code you wrote isn't doing what you want it to.