IOS16 was an IOS, installed and used exclusively by the Wii Backup Disc. When Nintendo went to fix the signing bug in all versions of IOS, they didn't think to release an updated version of IOS16, since it was never publicly distributed.
After that update, IOS16 was left as the one last IOS with the unfixed signing bug. It was then unofficially released and used to downgrade the System Menu prior to the 4.0 update, primarily to assist in piracy.
One strange property of IOS16 is that it still works on RVL-CPU-20, which made it a convenient target to downgrade other IOSes, although legal methods to downgrade were eventually released, making it simpler.
