[schmidtleonard]

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.

[upwardbound]

Do those charge pumps use external capacitors? If so, you could disconnect/float those capacitors, or if that would damage or glitch the chip, you could replace the real capacitors with some circuit like a voltage regulator + diode that would be designed to provide the charge pump output rail with a voltage that's high enough not to glitch the chip but low enough to be unsuccessful in writing to the flash. Would one of those ideas work, and allow the retrofit you envision to be designed with existing flash memory silicon + a few additional components?

[Scoundreller]

I believe they’re on-die capacitors for the charge pumps these days. The external ones are just for smoothening.

Probably a method to retrofit write/erase protection would be to just do power analysis and cap the current the flash chip can receive. Or shut it down if that works for you.

Not sure if they’re intelligent enough to run their charge pumps slower under compromised electrical conditions. Or if they’ll go haywire if they can’t do idle-time wear levelling/block erases.

[Scoundreller]

Isn’t it really only the erase that requires high voltage? So with a blank flash chip, without higher voltages, you could write to it to your heart’s content until you need to free up deleted content?

Edit: I think I’m wrong here and high voltage is needed for both.