Well… that was the upgrade from hell.
I think it’s all OK now. I hope. After completely rebuilding my WP installation from scratch and completely revamping my theme. I think we’re good to go.
Do let me know if you find something that’s broken or weirder than usual. And thanks for your patience.
Confidential to WP developers: Your opinion that something doesn’t make sense should not be taken as an indication that it’s OK to remove functions that users have come to rely on, or to make those function suddenly out of reach of those who aren’t comfortable hacking about with their templates. Nor is it OK to just willy-nilly rename stuff simply because it would be so much cleaner. And, yes, I realize that this is a labor of love for you. How wonderful. Working for a large company where that sort of no-warning impact to downstream users would get you fired might give you a slightly different point of view. It’s not about you any more. It’s about your thousands of clients who want things to just keep working.