The isolated switch circuit is insensitive to CTR because you only need to get enough current at the output to pull the FET's gate up against that pulldown resistor. Since the resistor is 15kΩ, it'll only take 800μA to get the gate all the way up to 12V (less the opto's VCE). With something like 17mA of forward current you need a CTR of only ~21% for it to work. Actually lower CTRs will work as well, depending on the threshold voltage of that FET and how much Rds you're willing to tolerate. Higher CTRs won't hurt anything, because the opto can't pull the gate up higher than the supply voltage. So it hardly matters what the precise CTR is. Higher CTRs can switch the FET on somewhat faster, but turnoff will be determined by the pulldown resistor regardless--and if you cared about switching speed you wouldn't be using this circuit anyway. This generally holds for all digital applications of optocouplers, it's pretty easy to design a circuit that works across the whole CTR range when it only needs to count to one.
Analog applications, as others have noted, require some sort of feedback. There are (or used to be?) some optocouplers that have one LED and two phototransistors in the same package that are designed for general-purpose analog isolation. The idea is that you'll get about the same output current from both phototransistors for a given input current, so you can use one of them as feedback on the sending side to adjust the LED drive current to ensure the signal on the other one is what you want. In practice the matching isn't necessarily great, though, so it's hard to get any degree of accuracy out of them.
One one project I tried setting an input threshold on the transistor side of an AC-input optocoupler, which is sort of the inverse of the above with two LEDs and one phototransistor, but even on those the CTR could be significantly different between the two polarities. Which polarity had the higher CTR varied between specimens, so it wasn't even just the construction (like one LED having slightly worse optical coupling to the phototransistor) that caused that difference.