You mean this? That’s about the only relevant thing I can find when searching for “ExpressibleByInteger”.
Again, I can’t help but contrast this with the Sequence protocols, which all have fairly specific requirements that enable a wealth of generic algorithms. None of which mandate literal support.
Perhaps you are overestimating my ability to find things. Could you point me to an explanation for that requirement? What about a reason that there's a static property for the additive identity in AdditiveArithmetic but not for the multiplicative identity in Numeric?