SGTM.  To confirm, this is only about removing the switch, correct?  (Not the ability for stars to override the bookmarks UI, which I think is still used by the extension?)

errr, I was going to remove everything. The extension was a project from many years ago which we never launched. 

The project did launch as a first party extension and has 860k users: https://chrome.google.com/webstore/detail/bookmark-manager/gmlllbghnfkpflemihljekbapjopfjik

You'll need to coordinate with the team behind that to remove support that's in use by the extension.

Something odd is going on. Their extension manifest.json has:
"""
  6    "chrome_ui_overrides": {                                                     
  7       "bookmarks_ui": {                                                                                                                                                                                                                
  8          "remove_bookmark_shortcut": true,                                      
  9          "remove_button": true                                                  
 10       }                                                                         
 11    },  
"""

However - those should have no effect unless the user has started chrome with a special switch. Is it because the validation code is missing an early return?

https://cs.chromium.org/chromium/src/chrome/common/extensions/manifest_handlers/ui_overrides_handler.cc?type=cs&q=enable_override_bookmarks_ui&g=0&l=156

The extension is whitelisted for access to that feature, among other things: https://cs.chromium.org/chromium/src/chrome/common/extensions/api/_manifest_features.json?rcl=3cc018ce0214d784aa6a1f2a6e6e70d7ccffcb7c&l=68-70

See also this code:
https://chromium.googlesource.com/chromium/src/+/8fc67b341c8d30821f9df226a769d97e86523f02/chrome/browser/ui/location_bar/location_bar.cc#65

    if (extensions::UIOverrides::RemovesBookmarkButton(i->get()) &&
        ((*i)->permissions_data()->HasAPIPermission(
             extensions::APIPermission::kBookmarkManagerPrivate) ||
         extensions::FeatureSwitch::enable_override_bookmarks_ui()
             ->IsEnabled()))
      return true;

We check if *either* the extension has the BookmarkManagerPrivate permission *or* the switch is enabled.

I installed the extension and it doesn't appear to be working properly. The images aren't populating. Furthermore, recent user reviews also seem to imply that it isn't being maintained...

Deleted: Screen Shot 2018-07-11 at 2.22.39 PM.png 665 KB

	Screen Shot 2018-07-11 at 2.22.39 PM.png 665 KB View Download

Deleted: Screen Shot 2018-07-11 at 2.23.40 PM.png 88.9 KB

	Screen Shot 2018-07-11 at 2.23.40 PM.png 88.9 KB View Download

I don't think it is being maintained.  I'd be supportive of fully deprecating and removing it, but we'll need to run it by more folks than just us, and likely not a discussion for this bug.

Right-o. Code isn't dead, so closing this bug.

We *could* still remove kEnableOverrideBookmarksUI (the switch) :)  I think that probably is "sufficiently dead".