Flash storage requires high voltage to do an erase (which needs to precede a write operation).

Back in the EPROM days, that was easy, just don’t supply 25V or whatever.

Modern flash still needs those high voltages but generates it on-chip via charge pumps. If your read-only switch physically disconnected the charge pumps, you would have read-only flash.

Writing flash takes (relatively) high voltage and the voltage boosting circuitry could be routed through a switch. It generally isn't, and the voltage converter is often an on-chip charge pump so this wouldn't be an easy retrofit, but the current state of affairs is due entirely to lack of interest rather than lack of possibility.

For years, Dell laptops came with a USB key containing drivers, a sort of "rescue boot disk". I once tried using them as a normal pen drive, but then realized they were read-only.

If there is a way to make them writable via software, that would be very interesting (and dangerous).

Erm. The read head and the write head in a magnetic drive are the same head. You can't keep the head away from the surface if you want to read the disk. But you can disable power to the driver that puts write current into the head.

... and you could absolutely build similar functionality into a flash chip. But most likely you can't actually buy such chips, at least not with any real capacity.