The PS/2 keyboard was originally an extension of the AT device. It supported a few additional host-to-keyboard commands and featured a smaller connector. These were the only differences between the two devices. However, computer hardware has never been about standards as much as compatibility. For this reason, any keyboard you buy today will be compatible with PS/2 and AT systems, but it may not fully support all the features of the original devices.
Today, "AT keyboard" and "PS/2 keyboard" refers only to their connector size. Which settings/commands any given keyboard does or does not support is anyone's guess. For example, the keyboard I'm using right now has a PS/2-style connector but only fully supports seven commands, partially supports two, and merely "acknowledges" the rest. In contrast, my "Test" keyboard has an AT-style connector but supports every feature/command of the original PS/2 device (plus a few extra.) It's important you treat modern keyboards as compatible, not standard. If your design a keyboard-related device that relies on non-general features, it may work on some systems, but not on others...