Unified Inbox in Outlook 2020?

 They all just go straight to the chosen inbox.

Interesting.  Does Sent mail and drafts also get unified into this "account"?

I assume we should create a new dummy email address to be the unification one, not an existing one which might get confusing?  What is your default maibox?  Mine is Microsoft 365 account so that all my devices (mac, ipad, iphone, windows pcs etc) get the same synchronized calendar and address book (I use my exchange account as the glue between all these systems, but I cant use outlook on windows with it ironically due to the missing (for 15 years) unified inbox/sent mail/drafts etc.

So far, it seems to behave exactly as a unified inbox.  Sent and deleted mail all goes to the same folders under the "unified" email address.

You could set up a dummy email, but it would only be to help your thought processes, not to manage the account.  Because I was experimenting, I used a generic address that I'd never used for anything, but you can still set your default email independently (as the "from" when you click an email link, for example), and Outlook still replies from whichever address received the message.  As far as I can tell, you could pick any email you have, for any reason that makes sense to you.  I could make an argument for using your default email or maybe, like me, you've got an old email that you keep mainly to use to fill in for forms that might make you a spam target or one that just doesn't do much.

An alternative to Thunderbird has been a recurring quest of mine for years, and I returned to it this evening.  The unified inbox thing has been a big hangup with Outlook, so I came across this thread.  After experimenting a bit, I seem to have found a satisfactory solution, but feel like I must be missing something, given the other comments and the fact that I didn't find this answer anywhere.

If I go to Account Settings and click on the Email tab, I see a list of all the emails I've added to Outlook.  At the bottom is a "Change Folder" button that corresponds with each email address as I click on it.  From what I can tell, I can use this button to send the mail from all of my email addresses to a single inbox.  So, I pick my default Outlook address, which doesn't receive any mail, and all of the emails from all my addresses go to that inbox.

Furthermore, it looks like as I go through those emails, if I hit reply, it automatically replies using the correct address.  

So, basically, whatever email address I pick as the default, I just send all addresses to that inbox, and that becomes my unified inbox.

Is there some reason this isn't a workable solution?

I just re-read this.  It seems like maybe MS has sneaked this in.  It seems like you can pipe mail into any box you want then.  For me I use emclient and I like the all inboxes, sent, trash, unread as unified boxes.  I can easily hop between 3 accounts and have nothing routed.  I am using an O365 account, a Gmail account and a hotmail account without any issues.

EMC just updated to release 8 and its gotten even better.  Further email is stored in a SQLlite DB and I can use the data from my client applications.  For instance I have a keyboard launcher that knows how to read my contacts table and I wrote an addin to search by name, then hit enter to dial, which launches the call on my Yealink IP desk phone.

I wonder if its time for another match with outlook?

You must only be using POP3.  There is no ‘Change Folder’ option with IMAP accounts.

Yes, although that's been my experience with unified inboxes generally, at least the way I've set them up before.