Apparently it's in iOS 8: https://support.apple.com/en-gb/HT204132

There doesn't seem to be an authoritative list for Android, but I did find this: https://android.googlesource.com/platform/libcore2/+/master/... -- not sure whether it means it's in the latest versions of Android, or whether that means it's going to be all versions of Android (e.g. with manufacturer/carrier modifications, etc) or what...