I was surprised too, but if you look at the timelines then RST_STREAM seems to have been present in early versions of SPDY, and SPDY seems mostly to have been designed around 2009. Attacks like Slowloris were coming out at about the same time, but they weren't well-known.

On the other hand, SYN cookies were introduced in 1996, so there's definitely some historic precedent for attacks in the (victim pays Y, attacker pays X, X<<Y) class.