It's upstream GRUB that's decided the older GRUB versions are insecure and not to be trusted. Microsoft just propagated that to machines running distros that weren't shipping patched GRUB builds yet. Up-to-date Debian wouldn't be affected provided that they downstreamed fixes quickly.
https://fedia.io/m/linux@lemmy.ml/t/1111595/-/comment/6916699 says that Debian's GRUB wasn't affected, but another part of the boot sequence was.
No, that is an entirely unrelated bad decision. It being okay to not have a popup to opt out of secure boot when it does its one job and notices you're about to run insecure code in kernel mode doesn't make every other user-hostile thing Microsoft ever does magically okay.