The beep was more an 'actual DOS interrupt event' happening from an invalid key sequence being generated, iirc.

The way a keyboard works internally, is through 'matrix composition'.

the number of matricies the keyboard uses, determines how many keys it can decode simultaneously, and if more than that are pressed, an invalid databyte is generated and sent. DOS's keyboard interrupt handler then issues the error beep.

The devil is in the details, but it's a combination of the limitations of that keyboard's matrix decoder, and DOS handler behavior.

DosBox does virtual input, so shouldnt be getting invalid sequences.

Modern keyboards have more complex matricies in them, and can handle more simultaneous keys.

So your saying DOSBOX overides the default keyboard repeat rate? I remember more... The arrow keys vs the numpad arrow keys, for a game I wanted to push 2 keys at the exact same time. Liero, to jump and toss the rope. Technically classical computers can only do 1 operation per time. I was on Windows 98se and changed the keyboard repeat rate either in the control panel or windows registry. I still have my old msdos computer given to me. Are there people that care to repair? May take it out and get pictures for fun, some day.... Opinion DOSBOX should not over-ride the default.

'Typematic Rate' could be set in DOS with a TSR, but usually was set in the bios in 386 and newer units.

but what I am referring to is a hardware limitation of old keyboards themselves.

They could not decode what keys were pressed when more than 3 were pressed at the same time.

As they improved over the decades, they can handle more simultaneous keys being pressed.