AFAIK notch is just declaration of intent, like with floppies and magnetic tapes - it's politely asking the reading device to not write to the medium, and it's up to the device to respect it (or up to user to not bridge the notch with a piece of tape).

Still, actual write-once (or read/write until hardware fuse is triggered, read-only afterwards) SD cards should be possible to make.

Since SD cards and USB sticks are both just computers you plug in to a network port on your computer, they could definitely make write-once SD card controllers.