So the funny thing is .. they didn't. They really didn't.

The original numbering plan for IP was that each network number became a /8. Which is how we miraculously ended up with 10/8, because network 10 was ARPANET itself, so 'flag day' left 10/8 vacant.

But back to the point at hand. IP itself is RFC 791, September 1981, nice and famous. The addition of classful networking because 255 network numbers wasn't going to last long, was RFC 790.

The first workaround for IP exhaustion was published before IP. It's been a Known Issue since day negative-one.