There is a high chance that there is no pyca/cryptography users on these niche platforms that Gentoo is trying to support. If there are any, they should pay for support not expect maintainers to support theirs fridge platform.
> The Gentoo/s390 Project works to keep Gentoo the most up to date and fastest s390 distribution available.
That's a declaration of support, and if that's not what they mean, they could list some limitations on their wiki. [1]
A lot of system distributions declare some platforms as supported and others as best effort and still others as probably not working, but you're welcome to try. That's reasonable, of course, but it's nice if you're upfront about it.
"was", because after this incident, careful review revealed that it isn't necessary, and dependency got removed. So yes, probably no users now.
I love this typo in context of s390!