Inadvertently developing on production happened.
We've had this change to categories in the works for a while now, and I was setting it up yesterday to deploy to our development server so some folks could test it. But... I forgot that I connected the development server to the live database to run the data import for our new site, Outdoors. When I set up categories on the dev server, the database migrations ended up happening on production, which left us with a database version ahead of the code version, and no easy way to roll the database back without losing data.
We had two options: either restore from backup and lose ~18 hours of data, including some new progress on Outdoors and some main Meta posts, or deploy the change to categories earlier than we intended and lose nothing but accept that the categories experience isn't as polished as it should be yet.
We went with the latter. So, hey, on the bright side: categories are here now! This is a change that should be useful for Writing in the near future. On the downside, it's not polished and there's some things missing, like the sort options and the lottery page. They're coming back, soon, when I have the time to move them into the categories UI. My bad!