The bootloader would still support SD, but the kernel would come up just fine without supporting it, as by the time the kernel is booting it's already in memory

The bootloader would still support SD, but the kernel would come up just fine without supporting it, as by the time the kernel is booting it's already in memory. You just wouldn't be able to mount any filesystems from that kernel, so you'd be stuck with whatever userland you packed into your initramfs