Zoom Slider on Keynote (Mac)

Do you mean for editing slides, so you can 'zoom in' on content on the slide for detailed editing?

You can achieve that via two-finger scrolling on your trackpad (assuming you're using your MacBook Pro as per your profile). Touch two fingers to the touch pad and spread them apart to zoom in, or pinch them together to zoom out.

It may be that since this functionality was added at a system level (this technique should work in most applications), the need for a separate control within Keynote was deprecated.

Off hand, since I normally use a MacBook Pro too, I don't know what the equivalent is for non-trackpad equipped Macs, but you can check System Settings -> Mouse or Accessibility -> Zoom for options.

I'm trying to think of what you mean by 'zoom slider' and am coming up blank.

What is it you're looking for/to do?