This has been reported by our users at: https://github.com/googlecolab/colabtools/issues/858.
The core code editor is based on: https://github.com/microsoft/monaco-editor
In Google Colaboratory we had a number of users who suddenly experienced keyboard shortcuts not working within the product. There were no corresponding changes or releases on our side. Users were able to resolve the issue by disabling the Evernote Web Clipper Chrome extension.
I see from https://chrome.google.com/webstore/detail/evernote-web-clipper/pioclpoplcdbaefihamjohnefbikjilc?hl=en that there was a release on Nov 11th, right around when users are reporting that this behavior in Google Colaboratory stopped working.
This extension release should be rolled back or patched ASAP. Please feel free to chime in on the bug at https://github.com/googlecolab/colabtools/issues/858.