eliz0x The SD slot can't be disabled every time the device is locked. That's not practical as it will be likely to corrupt the card sooner or later. Also potentially causing other issues if it is adopted storage.
Again, the hardware itself doesn't pose the threat. It's the software that accesses the hardware that potentially could run into trouble.
If the bootloader ignores a microSD slot, nothing in the slot can corrupt the bootloader. If the microSD device is used for adoptable storage, it can be 100% encrypted with a wrapped storage key, in which case swapping it out would randomize the logical device; this could be instantly detected. If it's not adopted then the situation is exactly the same as storage accessed via USB-C.
It is true that any connectivity potentially offers attack surface. But, again, people are generally happy to run video displays over USB-C, despite that being multilayered and quite complex. There isn't a rule that USB-C is safe while microSD is dangerous (or the other way around).