I was reading through the KornShell manual page and realized that most of the things that I love about bash originated in Korn. I also learned that bash (before version 4) couldn’t even touch the functionality of Korn. For example, Korn shell has associative arrays and floating-point math from the beginning.
The importance of KornShell is therefore that incorporating usage of all its interactive functionality translates across the boundaries of all shells from which they are derived. So it is still a horrible idea to bake Zsh completion habits into your fingers. It is always better to bake set -o vi instead since that will work on any modern UNIX/Linux system all of which come with at least one shell that is KornShell compatible.
As for scripting, POSIX compliant BourneShell is still best provided it is safely paired with shellcheck and shfmt. Such scripts will work on any UNIX or Linux system created since 1985. Applications that have higher performance demands should use dash for its minimalism and speed. Anything that makes extensive use of subshells should probably be written in another language like Python, Perl, or Go.