I tested it just now. As soon as the mailbox itself isn't tied to apps (i.e. Outlook for Desktop) by virtue of a license that "includes" the desktop apps, you cannot add it. So, that basically means their entire Exchange Online (which seems to be mostly the Basic users) user base (where users previously were able to add their account to Outlook, let's say if they had a Home & Business license), they can no longer do so going forward. It doesn't matter if they already have a separate, valid license for Outlook. So, in my scenario:
* I already have Microsoft Office 2021 Home and Business. Which grants me a license to use Outlook for Desktop. I have no "personal" mail accounts such as Gmail etc. So I have, at this point 1x Outlook license.
* I have ONE Business Standard license for my work e-mail (which gives me Exchange Online and grants me ANOTHER license to use Outlook for Desktop, on the same PC). So at this point, I have "2x" Outlook licenses. One occupied by this e-mail address, one left.
* I have ONE Business Basic license for a another e-mail (which gives me Exchange Online but NO desktop apps). So, at this point, I SHOULD be able to add this account because I have 1x Outlook license still left. But I can't. I can no longer add a second "business" e-mail, because "it doesn't have a separate (in my case, therefore, 3rd) license for apps (i.e. Outlook)".
The takeaway is: You can no longer add a mail account in the new Outlook if said mail account doesn't come with its OWN Outlook (apps) license.
This is ridiculous beyond understanding. Unacceptable to the point that if they don't fix this, I'll cancel BOTH Exchange licenses and move over to Google Business with my domains. Based on THIS interpretation though, they should therefore BLOCK adding 3rd party mail services, as they don't come with Outlook licenses! I'll give it a few days and then decide.