I dunno who did, but I'm quite sure that EVs came along shortly after the first release of pokemon.
I realized in yellow about EVs the second I saw Iron increase my defense stats; definitely someone would have been curious and tried to find out.
In Gen III the realized that the whole system revamped, and then was when I think the modern system of EVs was discovered.
IVs I seriously don't know. Maybe people had an idea of it before Gen III. This might support the fact that these numbers then also determined the shininess of a pokemon, and sites revealing this have been existing before Gen III came out.
Or maybe they found out when they grew curious about this 'Judge' who tells you about IVs, and decided to hack the game to find out.
But this is probably wrong, hence comment and not answer