The Zimbra Desktop is no longer supported. It will reach the end of Technical Guidance October 1, 2019.
So Desktop was canceled, then brought back, now canceled again? What is Zimbra planning to do in Desktop's absence?
As much as I love Zimbra Desktop, I would not use it with versions 8 or higher.
There's a bug between the SOAP version that it uses and the one that Zimbra 8 or higher uses. The bug is that the desktop doesn't use existing soap sessions. Each request ends up spawning a new soap session. On a low volume server, that's not a problem. But if you have multiple users using Zimbra Desktop, it means that the server can experience severe performance issues.
But what people loved about the Desktop product is how it had all the features which stock IMAP and POP3 clients don't have-- and that's your question: Where can I get those?
If you're a network edition customer, then those things (most of them, anyway) are within the NG ActiveSync module. Shared items, specifically.
But I'm afraid there isn't a 1 to 1 replacement for it at present. It doesn't hurt to let our team know that you want it, but we just didn't see enough depend for an offline version to keep it going-- but that isn't to say it's out of the question.