AGP used to be a big thing. It was cool too. You could power the GPU of your choice and with each year that AGP got faster you could still use your old cards! PCI GPU's were not the most popular and a few computers had PCI graphics slots, but AGP was super popular most of the time.
Uhh.. No? AGP had 2 or 3 versions, and they were not backwards compatible, not only the cards didn't work if you put them in an older slot, they fried up.
Manufacturers made them compatible with voltage supply settings in AGP 3.0 capable boards. But yes, if you stuck an AGP 3.0 card in a 2.0 slot you would end up with a dead board and GPU.
The standard AGP slots, no just the keying for them, some motherboards had keyless AGP slots, then there were the proprietary AGP standards made by a few of the motherboard manufacturers for server equipment.