It's not Bash. I purposefully stick to POSIX shell constructs because memorizing all the various Bash (and ksh and Zsh) extensions and their oddball semantics is too much trouble. By sticking to POSIX shell it's easier to learn and remember actual shell programming semantics--parameter expansion, positional parameters, pathname globbing, etc. You have to understand those things, anyhow, if you actually care to understand the language; they happen whether you know it or not, and they're fundamental to the language.
Plus, the POSIX specification for the shell is significantly more concise and clear than the manuals for Bash, Zsh, etc. See Shell & Utilities -> Shell Command Language at https://pubs.opengroup.org/onlinepubs/9699919799/ I know exactly where to go if I can't remember the difference between ${foo:-X} vs ${foo+Y}.
Plus, the POSIX specification for the shell is significantly more concise and clear than the manuals for Bash, Zsh, etc. See Shell & Utilities -> Shell Command Language at https://pubs.opengroup.org/onlinepubs/9699919799/ I know exactly where to go if I can't remember the difference between ${foo:-X} vs ${foo+Y}.