Hi @David H ,
Welcome to Microsoft Q&A!
As regards to your query about the On-Premise Exchange support in the New Outlook for Mac, I tried searching a lot within all the resources I can touch but cannot see it mentioned anywhere about the exact time of the availability of this feature.
Like you've mentioned, according to the official document available, the On-Prem Exchange Account Support is "Coming Soon"/"Upcoming":
The new Outlook for Mac
So, seems to me that the development is still ongoing and at present there is no estimated timeframe.
By the way, actually Outlook has an official feedback portal where customers can send feature requests or suggestions to the product team. I searched there and noticed that a similar idea has already been posted and the Outlook team has remarked that item as "We're working on this". I've added my vote there and I will leave the link below so that you or others see this topic can also vote it up or comment there.
https://feedbackportal.microsoft.com/feedback/idea/9cf3f9c5-75bd-ed11-83ff-002248273995
Hopefully this can be developed and rolled out to end-users soon.
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