It's been several years since I've dug into it and implemented the features I cared about for my personal shell in bash, but I recall that implementing a function to determine if an item exists in an array is significantly shorter and easier in bash 5 due to a feature that is not present in zsh. Probably too obscure for most people to care about, but at the time I was maintaining some rather preposterous bash glue code that relied on that, and compensating for zsh on some people's machines was tedious.