I agree with everyone about this being an utter-shambles, and if anyone has read the various pages of posts, you will see various by me expressing my frustrations.
I do think, however, that new posters need reminding, or pointing in the direction of the post I am replying to. Namely this URL:
https://www.microsoft.com/en-us/microsoft-365/roadmap?filters=&searchterms=375635
@Killing-me-MicroSoftly found this on the development roadmap, which states that they expect to be, finally, rolling out this feature by July.
If you read the roadmap item, though, you will see that it seems we will be required to manually add each Shared Mailbox to Outlook, if we want to utilise "All" the features of New Outlook.
This isn't a huge deal for me, as although I do have access to around 30 different Shared Mailboxes, there is only one that I need to be continuously monitoring. This means I would only need to add that one mailbox manually.
However, I think a lot of others out there will have many they need to perpetually monitor, so they will have to manually add each Shared Mailbox 🤦🏽♂️.
Moreover, as a Microsoft 365 reseller, it also means that I will need to manually add lots of Shared Mailboxes for my clients, if I want them to have all functionality, and not be continuously calling me asking why they cannot do X, Y and Z, Very frustrating, but at least they are, slowly, progressing with these basic features that should have existed since day one.
The other relevant link is this, which is where Microsoft rolled out the "auto-mapping" of Shared Mailboxes.
https://www.microsoft.com/en-us/microsoft-365/roadmap?featureid=164199
We can only hope that, eventually, the "auto-mapping" will simply give us full functionality on all mailbox types, without the need the manually add each.