This works as expected for me. I'm sure Hubitat has lots of built-in drivers that use it, but I'm using in a custom driver (for the Inovelli LZW36 fan/light switch) I wrote. I'm not sure what you're doing, but something like this:
myChildDevice.sendEvent([name: "supportedFanSpeeds", value: ["low", "medium", "high", "off"])
...should work (as should the named-parameter equivalent, though I'm not using it myself), where myChildDevice is a object referring to the device in question.
One issue is that this value doesn't seem to get used anywhere useful--notably, Dashboard still displays all the standard possibilities (or did last time I checked). An app could theoretically check these values and adjust its UI to match, but right now it's not clear to me how widespread this use is outside of anything internal to the driver itself (I suspect not at all).
