How do we create a keyboard without limit to simultaneous key presses?
Simple. We need PS/2 interface since USB interface has a limit (six to twelve keys) to simultaneous key presses. We need diodes for each key (about $2 for 200 diodes) to prevent ghosting. We need a driver that does not have anti-ghosting technology, a keyboard driver that does not register key presses during a potential ghosting event. This driver needs to be written from scratch.
Here's a link describing the keyboard matrix wiring under your keys. It describes ghosting problem and solutions to them.
The diode solution is useless if you have anti-ghosting technology. You need a driver that may support infinite simultaneous key presses.
Note that gaming keyboards use diodes or wire rearrangement for specific keys, W A S D, to ensure simultaneous key presses are valid, but they use anti-ghosting technology for the remainder of the keyboard.