How to design a keyboard circuit that can use a passive USB-to-PS/2 adapter?
Rather than follow the above comment success will more likely detect if the two pins are pulled up to 5V. (Make sure you use an MCU that has 5V compatible GPIO pins). PS/2 does run as open collector but in the immediate interval after the power supply is up to valid level the interface will be still inactive and both of the PS/2 lines will be undriven and high at 5V. This may require you to add some extra circuitry to detect this as opposed to some lower USB signal levels.

What is the incentive to support PS/2 in this day and age?

Added Information

The data sheet for your PIC16F1454 indicates that the connection of the PS/2 signal lines into the USB D+ and D- may not be advisable. The Absolute Maximum Ratings section has this to say about the D+ and D- pins:




With this information in hand it is not recommended to subject the D+ and D- pins to over 3.3V + 0.5V = 3.8V and certainly NEVER over 4.6V.