AGP cards and slots are keyed to prevent electrically incompatible stuff from being plugged together.
The Voodoo series, all being PCI chips on AGP cards, can only do PCI-style 3.3V signalling, 1.5V signalling as required for 4x/8x-only slots is not supported. Hence the missing 2nd key notch in its connector blade.
As you already noticed, 4x/8x-only boards have a key tab in exactly that position where the Voodoo card does NOT have a notch. This is to prevent you from inserting the incompatible card.
Boards that do not have the (expensive) voltage protection stuff on WILL fry if you insert an incompatible card - but for that to happen, the card would have to be incompatible AND wrongly keyed. There are some TNT2 cards that are falsely keyed 3.3V and 1.5V capable; the Voodoo range luckily is correctly keyed 3.3V-only.
Move on. Leave history behind.