What do you mean by valid? I don't see how the pack being out of print affects the validity of a scenario, otherwise everything in Beyond Valor, West of Alamein and Code of Bushido wouldn't be valid either.
Besides, I think of Streets of Fire as only sort of being out of print. You can still buy the unmounted boards from MMP for $5 a pop ($20 for the set) and the scenarios are downloadable for free. You don't get the AFV cards, but they were just a bonus from what I can gather.
I've played one SoF scenario recently as a learning scenario (my being taught by a player at our club) and it was a blast. In fact, it's the last one on that list... D1 "Guryev's HQ". Well worth a play. Of course, it didn't help I'm still hopeless in the city... :dead:... as I proved in a game of "Guards Counterattack" today against our newest memeber on the ladder!