Amethyst was inspired by XMonad, so it is probably the closest you are going to get to i3 on MacOS

Amethyst is nice but has odd and unfortunate edge cases. The use of the Accessibility APIs limits what it can do. For example, keybinds stop working if the focus is on an empty screen with no windows. Occassionaly it "loses" a window and you can't cycle to it any more, even if you focus it manually. I also don't think it can throw windows to other workspaces but maybe that's changed?

For very simple i3-like configurations (single workspace+monitor, always fullscreen or side-by-side), it's not bad. It's limited by poor support from Apple.

I3's scratchpad would be one. And title bars when windows are stacked or tabbed is another nice touch.