I have to ask, do you actually mean AS/400, or IBM i? Most current development for IBM i is done in PASE. I work in IBM i, and nobody I know is still writing C++ for the ILE. I imagine most of the ILE C++ is in maintenance mode and probably not using anything as modern as even C++20.
To be fair, idk about the software side of it. I just had to consider that they actually existed and were running stuff in the environment. However, one was a VM, at an auto repair shop, believe it or not.
Okay, but how many of them are going to have new software running on them built using C++31? If the toolchains are all stuck on C++11 (or more likely, C++98) then it doesn't really matter what C++31 will or won't do.