So anyone who buys new Green drives for a NAS is taking a bit of a gamble that they'll be usable?
Depends on your definition of "usable". If "usable" means "use the wdidle tool" then it is a gamble. In fact, for a short while there were some Greens made by WD that weren't compatible with the tool, but the tool was still actively developed by WD.
But the tool's ability to perform its function is totally dependent on WD. Clearly they've not tried to break it for 5+ years, so it's pretty clear what their intentions are. But there's no guarantees it will work on hard drives manufactured tomorrow. It's as good as "not supported" is for anything in the IT field. May work. May not.
You can bet money that if we have confirmed reports that the tool no longer works on Greens there will be a big warning on the front of this thread and I will stop recommending Greens altogether. As it is, the WD Reds aren't significantly more expensive than the Greens, so generally buying the Reds is the better action because of the warranty.