Is the Unreal Rigging Toolkit still being actively developed?

It’s not compatible with 4.17 and seems the main dev for it has left Unreal

It doesn’t appear to be in development at this moment in time, I’m afraid to say.

I looked into it for a university project several months ago, and found it to be lacking to be honest. I recommend that you look into building your own rigs; it’s not as difficult as you might first think!

If you’re serious about it, then I happen to know pluralsight offers a couple of decent rigging tutorials.
There are plenty off free ones on youtube too, however, which is where I learned how to.

Last commits where 2 years ago (aside from a copyright fix of 2017) but that does not necessary mean its not considered for further development but very unlikely I guess. Its just an “Extra” and there are planty of Tools for rigging out there as alternative.

Aside from that maybe they Plan to add something to Datasmith thats currently in Beta on the Rigging side for mentioned alternatives.