I have been a user of Bash for some time. I want to learn at least one other shell now, so I have been picking up the Korn Shell. However, it doesn't look like Ksh has had major updates for almost twenty years, and I have read that Zsh incorporates features from it. I have been reading many of the shell comparison questions on the StackExchange network, but have not found an answer to this: Are there features from Korn Shell that Zsh does not incorporate? If so, what are they?
Here's the Wikipedia article on command shell comparison.
According to that, the only feature ksh has that zsh doesn't is Named parameters for user defined "types". There may be others as that article is not an exhaustive list, especially for obscure features.
Going in the other direction, the answer to your question could fill a book (possibly this one).
Zsh has many features that ksh doesn't. A few popular ones are command stack, directory stack, history sharing and rprompt.
Here are some discussions on Stack Exchange sites about zsh: