Is this sort of breaking backward compatibility ? If stick strictly to semver, this should probably be in 4.0.0.

I just marked the property as deprecated, so this is not breaking backward compatibility. Removing it would break compatibility and warrant a 4.0.0 version.

But even when introducing breaking changes, it’s always a good idea to deprecate and explain with a comment what the replacement is.

Got it. Thanks for the explaination.

This looks like a major breaking change.

Before these changes the value of the property was set to NSUTF8StringEncoding = 4 in AFURLResponseSerialization's initializer, now it's defaulted to 0 which is actually an invalid string encoding.

If you try to encode/decode using 0 (e.g. [@"test" dataUsingEncoding:0]) you'll be prompted with the following warning:
Incorrect NSStringEncoding value 0x0000 detected. Assuming NSASCIIStringEncoding. Will stop this compatibility mapping behavior in the near future. 

So in the new implementation stringEncoding will be actually NSASCIIStringEncoding and not NSUTF8StringEncoding as it was before.