This document is 10 years old, and even then it acknowledged that finding shells without function support is a tough ask. You're not practically right, either.
When a system does not (correctly) implement portable standards, it is a bug in the system and software should not be corrected to accomodate for it. autotools disagrees with me on this point.
Yea, I'm going to side with autotools' maintainers on matters of software portability. And because I'm not aiming for autotools-levels of portability, I just write v3-compatible Bash, to help out my GPL2 OSX friends.
When a system does not (correctly) implement portable standards, it is a bug in the system and software should not be corrected to accomodate for it. autotools disagrees with me on this point.