Apparently FaceTime offers this. [1] Presumably Apple will allow other companies to do it as well, since they let them offer spatial audio in other contexts.
Why would Apple need to allow it? It's simply a matter of a given program postprocessing the various audio streams appropriately prior to muxing them for output.
Because Apple’s spatial audio varies as you rotate your head. The device knows where your head is located, but the app doesn’t (unless the OS tells it).
1: https://support.apple.com/guide/iphone/change-the-audio-sett...