See: https://web.dev/codelab-apple-touch-icon/, but once an app has been added to the home screen the apple-touch-icon never updates when changed.
<rdar://problem/68372709>
https://developer.apple.com/library/archive/documentation/AppleApplications/Reference/SafariWebContent/ConfiguringWebApplications/ConfiguringWebApplications.html
This is actually: <rdar://17555610>
The fix for this issue is needed outside the WebKit project, therefore this is being resolved as 'Moved'.