Last updated: 6/18/2018
6/18/2018: This feature is being slowly rolled out to Production beginning in 16.14 (18061302).
Original post:
With the release of the Insider Fast version ...
Updated Jun 18, 2018
Version 11.0
Blog Post
In case it helps anyone, here's a workaround I found:
1. First, create an ordinary IMAP account that does NOT refer to Google servers, but to some other provider. Let's call it mail.myprovider.com. If Outlook gets clever and tries to create an Exchange account, you have to click "Not Exchange?" at the top right to access the IMAP/POP option. This accomplishes two things: a) It removes the "authenticate with Google" popups completely and b) It enables display of "Advanced" options.
2. Then change the incoming server to imap.gmail.com or whatever you actually want. If you're now using a different username or password, click "More Options..." to set those. If you are NOT using smtp.gmail.com as your outgoing server, you will probably want to click "Advanced" then "Folders" then select the option to "Store sent messages in this folder:", otherwise they won't be saved anywhere.
Obviously this is all unnecessarily complicated - the price you pay for trying to mix poorly tested products from different vendors.