Set undriven outputs to VCC / GND

Setting unused pins as "outputs driving ground" has been the default choice in old Quartus versions. The reasoning in device handbooks was that the pins can act as additional ground connection to reduce ground bounce, which has some relevance for old packages like PQFP but is no longer reasonable for BGA packages with plenty of redundant GND pins. Although the additional ground connection is through output transistors, it surely shares some of the dynamic currents during switching.

But as Dave explained using the option is prone to damage FPGAs and external logic by shorting signals inadvertently.

It's not a big thing to set unused but externally connected pins to a specific level. Tri-state without weak pull-up (regular input) or tri-state with bus hold could be a safe alternative to driving it low.

The weak pull-up feature of unconfigured pins suggests to define all outputs active low rather than active high, by the way. Doing so, an unused but externally connected pin will be inactive without specific measures.