cool. thought so. thats wild.. I mean eye tracking is stellar in visionOS - but the fact you cant make anything that uses it is.. wild.. head tracking is prolly better done on MacOS.. but I hear you. Filed a enhancement request

No answer to this really but original docs date to 2014 https://web.archive.org/web/20140604054226/https://developer.apple.com/library/Mac/documentation/TextFonts/Reference/TextInputSourcesReference/Reference/reference.html#//apple_ref/c/func/TISGetInputSourceProperty