After installing the latest update of Mac OS X macOS (macOS 10.12, Sierra), I noticed that my hotkey for iTerm2's slide-down window was not working anymore.
Turns out that Karabiner has a known issue with macOS Sierra and does not work any more.
The developers are already developing a fix and in the meantime provide Karabiner-Elements as a hot-fix package. From what I understand, the functionality of the KE is limited to single key remapping, but can be combined with Keyboard Maestro/Better Touch Tool for more sophisticated shortcuts (just search the GitHub Issues of Karabiner/KE).
KE does not come with a (useful) GUI interface and can be configured via a JSON file.

iTerm's hotkey is configured to be the F18 key, so I need to my Caps-Lock key (WHO NEEDS THIS KEY ANYWAY) to this key.
I configured it in Karabiner already and it also doable in Karabiner-Elements.
The following is my karabiner.json:

    "profiles": [
            "name": "Default profile",
            "selected": true,
            "simple_modifications": {
                "caps_lock": "f18"

Next, I have assigned F18 to the Hotkey Window profile in iTerm:

F18 mapped to the Hotkey Window profile

It's all working again!

(picture credit: Toby on Flickr)