Comment 6 for bug 137136

I note that (a) /etc/default/grub isn't documented in the update-grub(8) manpage; (b) there's no indication that this supports_quiet setting is supposed to be a user setting rather than a random internal script variable that might stop working or behave differently in future (c) there's no way to have some kernels have the grub 'quiet' setting but not others using this workaround.

I don't think I agree with your reading of the use case either. The text of that page up to the use case bits is talking generally about all messages printed before the usplash kicks in, and to the extent that it distinguishes between grub's 'quiet' and the kernel's quiet, it's talking about the former, not the latter.

So I disagree with this bug being closed as 'invalid'.