ASTC encoding on iOS

I found following code for ASTC encoding:

Code Block NSMutableData *data = [NSMutableData data];
CGImageDestinationRef destination = CGImageDestinationCreateWithData((CFMutableDataRef)data, (CFStringRef)@"org.khronos.ktx", 1, nil);
NSDictionary *properties = @{@"kCGImagePropertyASTCBlockSize": @(0x88)};
CGImageDestinationAddImage(destination, source, (CFDictionaryRef)properties);
CGImageDestinationFinalize(destination);

Code Block
NSMutableData *data = [NSMutableData data];
CGImageDestinationRef destination = CGImageDestinationCreateWithData((CFMutableDataRef)data, (CFStringRef)@"org.khronos.ktx", 1, nil);
NSDictionary *properties = @{@"kCGImagePropertyASTCBlockSize": @(0x88)};
CGImageDestinationAddImage(destination, source, (CFDictionaryRef)properties);
CGImageDestinationFinalize(destination);

This code produce data in KTX format. I can not find any documentation about kCGImagePropertyASTCBlockSize parameter name so I consider that this code is undocumented.

I have two questions:

Is it safe to use it? May Apple change this in future?
Is it hardware encoding or CPU encoding?