tldr; chunkwm has been completely rewritten and is now yabai

From the chunkwm site:

chunkwm is no longer in development because of a C99 re-write, yabai.

yabai was originally supposed to be the first RC version of chunkwm. However due to major architectural changes, supported systems, and changes to functionality, it is being released separately. There are multiple reasons behind these changes, based on the experience I've gained through experimenting with, designing, and using both kwm and chunkwm. Some of these changes are performance related while other changes have been made to keep the user experience simple and more complete, attempts to achieve a seamless integration with the operating system (when possible), proper error reporting, and yet still keep the property of being customizable.

For those who don't know, chunkwm was / is a tiling windows manager that is sort of like bspwm / i3 etc. I've been using chunkwm for a few months now and love it. If you're also an i3 user, the lack of a proper super key does make your key combos different, but overall its an excellent window manager. Both chunkwm and yabai use koekeishiya's Simple Hotkey Daemon (skhd).

Anyway, I gave the new version the day and its pretty good, but still has some quirks. It seems like development is moving along quickly, so keep an eye on it.