At least plymouth and probably every init system out there.
Linus said that "we very much expose /proc/cmdline for a reason. System services are supposed to parse it, because it gives a unified way for people to pass in various flags. [...] And yes, that does include "quiet" and "debug". Parsing them and doing something sane with them is not a bug, it's a feature."
http://lkml.iu.edu/hypermail/linux/kernel/1404.0/01488.html
> The bug report makes it immediately clear that this is not really about a technical problem that ought to be solved - no, the dirty user-space programmers should get their stinking paws off of our good kernel command line flags, how dare they!
You're right that Kay was wrong to close the bug immediately instead of trying to help finding the real cause, and you're wrong when you say that userspace should not parse the kernel command line, see Linus' remark above.
> There's no reason to even assume procfs will always be there
To the contrary, dropping it would be an ABI break so it's really safe to assume that it will be always be there.
> who's been programming for more than an year
Please, don't lecture others about their job. I'm not assuming you're an incompetent fool, please don't assume I am.
> if a program did that on Windows
And can we try to stay relatively on topic? Windows is definitely unrelated.
(I'm not otherwise replying in detail because you assumed that parsing the kernel command line was not ok, while Linus officially blessed its usage. Also we reached the max thread depth on HN, which should probably suggest something.)