Seems like there are different interpretations by authors/maintainers of what is required versus what is optional.
As an ordinary user, I am most interested, perhaps mistakenly, in POSIX for a single reason: portability. (Whether portability is a goal of POSIX I am not sure. I have not done much research. Maybe it isn't.) As a user, I want to be able to write scripts on Linux than run on BSD and vice versa. Perhaps I have conflated portability with "POSIX compliance". However, as a practical matter, I would not use these operators in scripts that I wanted to be portable. When I have the motivation, I am working on a unenhanced port of NetBSD sh to Linux, i.e., without the Herbert Xu changes. Being lazy, so far I just added tab completion to dash so it feels more like NetBSD sh.
The "Bible" I would be interested in reading, if it exists. is the "Portable Sh Bible". (I am not a Bash user.) When I have a question I usually consult https://www.in-ulm.de/~mascheck/
