It turned out that this keybinding was in my file. It looked like this:

{
    "key": "ctrl+backspace",
    "command": "deleteWordLeft",
    "when": "textInputFocus &&  && !editorReadonly &&inlineDiffs.activeEditorWithDiffs"
},

Quite unnoticeable, isn't it?
The problem is the condition syntax error: ... textInputFocus && && !editorReadonly .... This condition is considered absent and the keystroke cannot be removed due to uncompleted when expression.

The "moral" of this is that syntax errors in keybindings.json file can lead to unpredictable results.