My guess is that you've upgraded macOS, which in turn has caused bash to downgrade from 5.x to 3.3.
On Catalina bash is actually 3.2.57, it appears. Why on earth ... oh well.
ksh is at least from 2012, could experiment running the pre-zsh-ized scripts with that.
Apple can't (or rather won't) use newer bash than 3.2.57 due to licensing woes (GPL3) that's why they want users to switch to zsh and why macOS 10.15+ now use zsh by default for new users.
So while I DID upgrade my main machine I doubt that Apple downgraded my bash.
I'll have to try with a different machine with an older macOS to verify that on those this all behaves similar