Interesting, but I guess it's not the full story.
Now Playing indeed suddenly disappeared from the Modular watch face according to Apple Watch faces and their features - Apple Support (MK).
But there are other watch faces that still support this Now Playing complication, e.g. Nike Compact. This is exactly the same complication and I can still create a watch face with it. However, Nike Compact doesn't show the currently playing song either...

It also doesn't show my HomePod's name but it says (null)...
I still hope it's some kind of bug. It was really nice to just glance and see what's playing instead of having to resort to Smart Stack which is a button press.