Configuring preferences and options in Outlook.com
You can either modify the registry or using 3rd party tool. If you choose to modify the registry route then please make sure you back up the registry first. The 3rd party tool offers 60 days trial which is ideal if you only want to delete the search history once.
Please refer to this article: https://www.extendoffice.com/documents/outlook/...
DISCLAIMER:
There's a link above from the non-Microsoft website. The page appears to be providing accurate, safe information. Watch out for ads on the site that may advertise products frequently classified as a PUP (Potentially Unwanted Products). Thoroughly research any product advertised on the site before you decide to download and install it.
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