I actually did dive back to this old post to try to delete it so that there wouldn’t be this confusion, but the post was already too old and permission to modify/delete had been removed. But shaped windows were not gone for long.
At least we can all say that shaped window is back in a slightly new form. In my opinion this version is more stable than what we had in SDL2, see the second reply here where I wrote some minimal code to draw a shaped window using a PNG image. (It’s already slightly outdated as the SDL3 source has continued to evolve).