Remember when Coca-Cola changed their formula, and everyone went nuts, and then they came out with Coca-Cola Classic?
RS was changed, the main difference being that the front of the cutter got a wavy profile that increased the ramp of the depth gauge, making it a more reduced kick back style of chain. Then everybody went nuts, so STIHL came out with RSK, the K stands for "Klassic". So RSK has the straight front like RS had years ago. So it may create more kick back energy in a kick back event, but it also cuts a little more aggressively than the newer RS, although both are considered yellow family chains, meaning not reduced kick back chains. Green family, such as PMN, PM1, RM2, and RSC3, are considered reduced kick back chains, and I prefer them for that reason. They really are not that much slower than any other style of chain, if kept sharp and with the correct depth gauge.
RSC is the newer style RS with the comfort feature, which reduces vibration. It works well, as I have cut with RM2 (green reduced kick back chain), RS and RSC one right after the other, and RS is a little, not much, faster than RM2, and RSC is about as fast as RS, but much smoother. I have not done this timed test with RSK, though. But as you have noticed, RS is being replaced by RSC, but RSK should still be available.
RM2 is being replaced by RSC3, and I am a little sad to see the RM2 go, since it is a semi-chisel cutter, and doesn't get as dull as fast as RSC3 will.