5V is dead, and has been for years. Even 3.3V is dying. These are among the lowest power chips in the world, regular bleeding edge stuff. Higher voltage mean

5V is dead, and has been for years. Even 3.3V is dying. These are among the lowest power chips in the world, regular bleeding edge stuff. Higher voltage means higher power, so there is a race to get as low as possible. Internally, the D21 core runs at 1.2V. (There's an internal regulator that needs at least 1.62V coming in.) But anything above that is just a waste of energy (besides the fact that the highest clock speeds need 2.7V minimum for 0 or 1 wait states). They allow it, for compatibility, but you can see why even 3.3V is dying. The shields will be happily redesigned.