Linus has said earlier that this rule has been in place pretty much since day 1 (in a google+ thread here: https://plus.google.com/115250422803614415116/posts/h

Linus has said earlier that this rule has been in place pretty much since day 1 (in a google+ thread here: https://plus.google.com/115250422803614415116/posts/hMT5kW8LKJk https://plus.google.com/115250422803614415116/posts/hMT5kW8L...), although he's made it much more explicit than it was at first.