AIR, this was a system we actually kept in AD&D, which, frankly, does explain why everyone had an 18 in their main stat.

But, even in other groups, or in pre-gen characters, it wasn't exactly rare. Like I said, all you have to do is start tracking the stats - sure, you might have the odd character that doesn't have an 18 in their main stat, but, that's likely the odd one out. Heck, the standard array in 5e wouldn't even make a playable character in 1e. 15, 14, 13, 12, 10 8? That's a reroll waiting to happen.