In an interview with a Swiss newspaper, Apple executives who work on watchOS,Kevin Lynch and Deidre Caldbeck, basically said that there will not be official third-party watch face support in watchOS anytime soon.
9to5Mac has a summary of the article here: https://9to5mac.com/2023/06/19/interview-watchos-10-third-party-apple-watch-faces/
This actually seems like a good argument to me, though I suppose it's also possible to design a watch face API that requires a minimum API version and ignores a watch face made with an older version should Apple improve an API in a way that prevents older watch faces from working.
Asked about the possibility of third-party watch faces on Apple Watch, Lynch says that the watch face is the home screen of the watch and they want everything to work reliably and consistently.
As Apple controls all the faces the user can choose, Caldbeck adds that users “don’t have to worry about the watch face still working when there’s a major watchOS update. We’ll take care of that.”
If third-party faces were available, Apple argues it wouldn’t be able to ensure that the watch faces keep working if they change something in the operating system, like this year’s watchOS 10 redesign that includes a new swipe up gesture to reveal a tray of user-selectable widgets.
9to5Mac has a summary of the article here: https://9to5mac.com/2023/06/19/interview-watchos-10-third-party-apple-watch-faces/
This actually seems like a good argument to me, though I suppose it's also possible to design a watch face API that requires a minimum API version and ignores a watch face made with an older version should Apple improve an API in a way that prevents older watch faces from working.