I want to take this issue. :)

The CL is ready to review. (https://chromium-review.googlesource.com/c/chromium/src/+/1185337)

thanks, i wasn't able to find the file in question that controlled this via cs/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/fe6597ef11c02e3dac3bd26650b90ba71d8c37af

commit fe6597ef11c02e3dac3bd26650b90ba71d8c37af
Author: Mike Frysinger <vapier@chromium.org>
Date: Wed Aug 22 20:22:53 2018

shortcut_viewer: document Launcher+Backspace as Delete

We want to push people to use the Launcher as the modifier everywhere.
This particular Delete shortcut was missing as Launcher+Backspace is
already wired up to be Delete.

Bug: 876774
Change-Id: Ibf1569a5094f819451ce32fe2178ed68fca9e830
Reviewed-on: https://chromium-review.googlesource.com/1185213
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Tao Wu <wutao@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585226}
[modify] https://crrev.com/fe6597ef11c02e3dac3bd26650b90ba71d8c37af/ash/components/shortcut_viewer/keyboard_shortcut_viewer_metadata.cc

Will we phase out Alt+Backspace over a milestone or two as with other keyboard shortcuts, and show a notification when the user uses the deprecated shortcut?

that would be nice, although i have no idea how to implement that :)