The answer to the question is, unfortunately, very clear: native Outlook compatibility using the MAPI (a.k.a. Exchange) protocol via OpenChange has been dropped indefinitely, as stated in the release notes of Zentyal 5.
The fact that no one from the Zentyal project has commented on the reasons for this, and that
www.openchange.org is now unreachable, is quite annoying for me, and lets me think something happened behind the curtains. We all know OpenChange was not production quality, but this looks too sudden to me. Moreover, the "native Outlook" category in the SOGo bug tracker has been marked "(obsolete)", even if I can't find references anywhere else.
And yes, Zentyal marketed itself as THE Exchange replacement, pointing out that what differentiated them from other similar (e.g. Nethserver) projects was the native Outlook support.
All in all, I do not think Zentyal makes much sense now. Rolling your own solution based on vanilla Ubuntu 16.04 is almost trivial, especially with some help from Zentyal configuration files. I've done that and I'm quite satisfied. And I also installed OpenChange from the SOGo repositories, although I've then disabled it because of some glitches (but they were there in Zentyal 4 too...).
Using Outlook via SMTP and IMAP, the open source Outlook CalDav Synchronizer plugin is good enough to fill the hole for contacts and calendars (shameless plug: I'm the author of the backport to .net 4 for Windows XP compatibility).