At some point in the past years I was able to find notebook content via Windows Desktop Search (WDS). Recently I've had trouble with this (null results searching for content and "ext:nb"). I checked WDS Indexing Options, and found a problem with file extensions "cdf", "nb", "nbp": "Registered IFilter is not found". Extensions "m", "ma", and "wl" appear to not have such a problem, with WDS reporting "Plain Text Filter". I notice this after a clean install of both v11.3 and v12.
If the relevant IFilters are indeed installed on my system, perhaps my company's IT security policies are preventing their proper registration. I ran the installers as admin, but perhaps that wasn't enough.