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Just replace black boxes with '#' and voila you have real ASCII.

It's not that simple. You have also half blocks (U+2580 UPPER HALF BLOCK, U+2584 LOWER HALF BLOCK) to deal with.

Full: # @ % O Z

Upper half: * ' " ` ^

Lower half: o _ z

Source: have played nethack.

My phone correctly read the code from my laptop screen as-is. I don't think it would with actual low-byte characters.

It probably would if you use the “unreadable” xterm font haha.

Why? I've had my phone read QR codes made of entirely the '#' character, albeit with a small fixed-width font.