As someone with no skill in these types of games and even less knowledge of the current market, which major developers are still working on RTS-type games and which ones have died out?
Well of the top of my head (and maybe including a couple of not quite so major devs):
Some of the previous but now dead developer are:
Ensemble Studios - Age of Empire & Mythology series
Westwood Studios - Command&Conquer series
Stainless Steel Studios & Mad Doc - Empire Earth 1 & 2
PopTop Software - Tropico 1 & 2 (merged with Firaxis)
Bullfrog Productions: Numerous (Populous, Dungeon keeper)
Big Huge Games - Rise of Nations
Cavedog Entertainment - Total Annihilation
Ascaron Entertainment - Patrician, Port Royale
Dead to me:
Firefly - Stronghold series
Probably dead to others now:
EA Los Angeles - (LotR: Battle of Middle Earth, Command & Conquer/Red Alert 3, Medal of Honor (now Danger Close Games))
Still alive:
Related Designs - Anno series
Haemimont Games - Various (Tzar, Grand Ages, Tropico 3 & 4)
TimeGate Studios - Kohan (never played it, they make FPS games now (FEAR, Section 8)
Best Way + Digitalmindsoft: Men of War
Blizzard: Starcraft, Warcraft
Ubisoft Blue Byte - The Settlers
Eugen Systems - RUSE
Relic - Numerous (Homeworld, Company of Heroes, Warhammer 40k)
Paradox Interactive - EU, CK, HoI, etc
Stardock: Sins of a Solar Empire
Creative Assembly: Total War series