This prompted me to fire up Mario Kart for the first time in a while, and although the Wave 6 courses aren't available yet I did receive Wave 5 and possibly Wave 4. Hard to tell, since they all look so similar and have mostly come from older Mario Karts.
I wonder what the reason is for spreading the DLC over two years. Something to make it seem like they're taking care of the online subscribers, perhaps? I bought the whole thing up front, so it has been a poor service from my perspective.