You have things backwards. It was the systemd developers that didn't want the debug command line to be used against systemd. They were the ones who filed the patch to prevent the debug command line from reaching user space (/proc/cmdline) and it was the kernel developers (well, Linus specifically) who refused to patch and demanded that systemd cleaned up their debugging output so that 1) it didn't crash Linux (which it was doing previously), and 2) the output was more useful to end users for debugging.
Yeah, that was an odd fix. Then /proc/cmdline doesn't reflect what the actual kernel command line was, thereby making debugging more opaque. The sane option, which the kernel developers suggested, was that systemd namespace pre-boot options it takes from the kernel command line, just like every kernel subsystem already does.
