I can't seem to find the achievement for carving 500 pumpkins in my Morostide section (I don't have it, but I thought it was supposed to show up regardless grayed-out).

Is it retired, or unavailable until next Morostide...?

I still have it in mine. With tiered achievements you only see two of them The last one you got, and the next one you need. I see it because its the next one I need.