I was working towards this achievement, fed my pet a storm cookie and now regretting it because I can't see the progress anymore in the achievements section. Has it retired now?

not my knowledge no, it could be a cache issue though

The achievement won’t retire, and the cookies are still going out until the end of the month.

If you type "20th" in the search bar in the upper left of the achievements page, it will bring up the cookie achievements.