I have noticed that a lot of the features listed in https://www.gnu.org/software/bash/manual/bash.html#Major-Dif... aren't present in zsh, but I am not sure of all the ones that aren't in zsh.

Ones that I have used in bash that aren't in zsh (there may be many more, I stopped using zsh in many scenarios because of some of these):

* Some of https://www.gnu.org/software/bash/manual/bash.html#Shell-Par... (e.g at least ${LOGNAME^^}, `(FOO=BAR;echo ${FOO,,})`)

* -p option to read for the prompt, e.g. `read -s -p "Enter the DB password: " PW`

"Zsh is able to emulate POSIX shells, but its default mode is not POSIX compatible, either."

from http://zsh.sourceforge.net/Doc/Release/index.html