As of 08/19/21, when calling this new API writeHEIF10RepresentationOfImage with the same arguments as writeHEIFRepresentationOfImage (minus the forum argument), I get the following exception...
*** -[__NSPlaceholderDictionary initWithObjects:forKeys:count:]: attempt to insert nil object from objects[0] (NSInvalidArgumentException)
I am assuming that the new API isn't working, but I'm posting here for visibility.
I used Xcode 13.0 Beta 5 and I tested it on an iPhone 12 running iOS 15.0 Beta 6