There are actually a handful of apps that do use the API, including a couple of component apps (e.g the music player used by the Files app).

I can never remember the details --- aren't there two different panels APIs?

this[1] says "'panel' is deprecated and only available to existing whitelisted extensions on Chrome OS." In the code here[2] it certainly looks like it's only whitelisting hangouts extensions, which we can probably go ahead and remove, right? That would allow us to get rid of a bit of code like AshPanelContents.

[1] https://developer.chrome.com/extensions/windows
[2] https://cs.chromium.org/chromium/src/chrome/browser/extensions/api/tabs/tabs_api.cc?rcl=0&l=506

We eliminated panels entirely from the Desktop (non CrOS) platform.

We no longer support panels in the extension api (chrome.window). We should update the doc and consider eliminating the hack for the no-longer-supported Hangouts extension (although removing that hack would make some of us sad).

We still support panels in the app api for CrOS only (chrome.app.window). This is what, e.g. the music player app uses.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a3fef705d9ca89bff862e80046780b02b093d81c

commit a3fef705d9ca89bff862e80046780b02b093d81c
Author: estade <estade@chromium.org>
Date: Thu Dec 15 01:11:46 2016

Remove unused IDR_PANEL_* assets.

BUG=522168, 672669 

Review-Url: https://codereview.chromium.org/2573903003
Cr-Commit-Position: refs/heads/master@{#438693}

[delete] https://crrev.com/6a02cc8f81da40e6829d1bf1c484cf05299ba9e0/chrome/app/theme/default_100_percent/panel_close.png
[delete] https://crrev.com/6a02cc8f81da40e6829d1bf1c484cf05299ba9e0/chrome/app/theme/default_100_percent/panel_close_click.png
[delete] https://crrev.com/6a02cc8f81da40e6829d1bf1c484cf05299ba9e0/chrome/app/theme/default_100_percent/panel_close_hover.png
[delete] https://crrev.com/6a02cc8f81da40e6829d1bf1c484cf05299ba9e0/chrome/app/theme/default_100_percent/panel_minimize.png
[delete] https://crrev.com/6a02cc8f81da40e6829d1bf1c484cf05299ba9e0/chrome/app/theme/default_100_percent/panel_minimize_click.png
[delete] https://crrev.com/6a02cc8f81da40e6829d1bf1c484cf05299ba9e0/chrome/app/theme/default_100_percent/panel_minimize_hover.png
[delete] https://crrev.com/6a02cc8f81da40e6829d1bf1c484cf05299ba9e0/chrome/app/theme/default_100_percent/panel_restore.png
[delete] https://crrev.com/6a02cc8f81da40e6829d1bf1c484cf05299ba9e0/chrome/app/theme/default_100_percent/panel_restore_click.png
[delete] https://crrev.com/6a02cc8f81da40e6829d1bf1c484cf05299ba9e0/chrome/app/theme/default_100_percent/panel_restore_hover.png
[delete] https://crrev.com/6a02cc8f81da40e6829d1bf1c484cf05299ba9e0/chrome/app/theme/default_200_percent/panel_close.png
[delete] https://crrev.com/6a02cc8f81da40e6829d1bf1c484cf05299ba9e0/chrome/app/theme/default_200_percent/panel_close_click.png
[delete] https://crrev.com/6a02cc8f81da40e6829d1bf1c484cf05299ba9e0/chrome/app/theme/default_200_percent/panel_close_hover.png
[delete] https://crrev.com/6a02cc8f81da40e6829d1bf1c484cf05299ba9e0/chrome/app/theme/default_200_percent/panel_minimize.png
[delete] https://crrev.com/6a02cc8f81da40e6829d1bf1c484cf05299ba9e0/chrome/app/theme/default_200_percent/panel_minimize_click.png
[delete] https://crrev.com/6a02cc8f81da40e6829d1bf1c484cf05299ba9e0/chrome/app/theme/default_200_percent/panel_minimize_hover.png
[delete] https://crrev.com/6a02cc8f81da40e6829d1bf1c484cf05299ba9e0/chrome/app/theme/default_200_percent/panel_restore.png
[delete] https://crrev.com/6a02cc8f81da40e6829d1bf1c484cf05299ba9e0/chrome/app/theme/default_200_percent/panel_restore_click.png
[delete] https://crrev.com/6a02cc8f81da40e6829d1bf1c484cf05299ba9e0/chrome/app/theme/default_200_percent/panel_restore_hover.png
[modify] https://crrev.com/a3fef705d9ca89bff862e80046780b02b093d81c/chrome/app/theme/theme_resources.grd

Is there anything else that should be removed / should the API be deprecated for non-component apps? Otherwise feel free to mark this as fixed.