>Some comments in no particular order:
>
>1. For me, Outlook only shows address book(s) "in a different interface" if I click on the "To" button to the left of the field itself. If I start typing an address directly in the To field, I get a "Recent People" dropdown list below the field itself, which gets filtered as I type

Exactly, this is how it was working until yesterday.

>2. If I do click the "To" button, I can select the address book I'd like to search at the top right. If you're doing that you may, by default, be looking at a different address book than usual

But, point 1 shows that something is different.

>3. Microsoft Office did install an update for me yesterday. The behaviour above is what I'm seeing today, I don't see any difference from what I had earlier

Ok, good

>4. If you've recently created a new user profile on your computer, or a new Outlook profile, by default you will lose your previous autocomplete list/cache unless you take steps to preserve and restore it: https://docs.microsoft.com/en-us/outlook/troubleshoot/contacts/information-about-the-outlook-autocomplete-list . If you don't do this, autocomplete will eventually re-learn and become useful again

Before, when typing, a list was appearing. On that list, I was able to click X to remove one from history. This is also gone. I wonder what happened.