Mail & Calendar Apps Will Be Replaced with New Outlook for Windows December 2024

The need to run both was due to the loss of access to my Outlook.com email account through the Outlook 2013 - it ceased to log in and I could not find any way to get an App Password or other credentials to allow me access, hence the need to use the New Outlook. If there is a way to regain this access I would be very appreciative.

Correct. Outlook 2013 does not support oAuth2 and Microsoft now requires it to connect. Email does not support app passwords but they are available for other apps, like xbox that does not support oauth. (Need to enable 2FA to create them).

I have been trying, without success, to rewrite application to see the New Outlook but that application does  not appear in the Registry nor in the Programs and Features list though it does appear in the Task Manager Task list. Some forum posts say that the New Outlook is not discoverable and does not have an API - is that true? 

True - new Outlook can't be automated. 

The New Outlook places an advertisement in the Outlook inbox so that even if there is no real email in there it shows that there is an unread email, which is not fair and is a waste of time on a massive scale over the user base. Apparently, none of the other non-Microsoft accounts do this, for which I am grateful.

If you have a subscription and the Microsoft account is set as the primary account, there are no ads. From what I understand, if no subscription, all accounts should have ads at the top of the inbox. 

Tip: if you use Focused inbox, the ads are only shown on the Other tab. 

The ability to open .PST files does not appear to be a functionality of the New Outlook. If I am wrong, please point me to that functionality.

It's in new Outlook now but the PST files ae read-only at this time. Look in Settings > General > Outlook Data Files - but it appears to only be listed in my Outlook when a Microsoft mailbox is set as default.