I found a work-around. Credit goes to Azylog on github for most of this.https://github.com/ZeXtras/zimbra-drive/issues/23
Pretty simple : just change lines :
class OC_User_Zimbra extends \OC_User_Backend
class OC_User_Zimbra extends \OC\User\Backend
In 3 files : oc_user_zimbra.php, abstractzimbrausersbackend.php, zimbrausersbackend.php.
I also had to disable autoload check in lib/autoload.php :
// throw new AutoloadNotAllowedException("$fullPath $root");
because it was failling with :
OCP\AutoloadNotAllowedException: Autoload path not allowed: /var/www/html/nextcloud/apps/zimbradrive/lib/auth/zimbrausersbackend.php in /var/www/html/nextcloud/lib/autoloader.php:134
Only one of those files had that exact line to change so I just searched for \OC_User_Backend in each file. There is a single instance of each, change \OC_User_Backend to \OC\User\Backend. Those files are all under /apps/zimbradrive/lib/auth of your Nextcloud directory.
For the other one, the file is in the lib folder directly in the Nextcloud directory. It's not in lib in the zimbradrive folder. And the file is autoloader.php not autoload.php. Go to line 134 and comment out "throw new AutoloadNotAllowedException("$fullPath $root");" and add a "return true;" after.
Once that is done you need to go into /apps/zimbradrive/appinfo and find the info.xml file. Search for "<nextcloud min-version="9" max-version="12"/>" and change the max-version="12" to max-version="13". Lastly, I had to run "sudo -u apache php occ app:enable zimbradrive" from the nextcloud directory to re-enable zimbradrive in Nextcloud.
This will get you going again if you updated without thinking like me. If you haven't upgraded yet, I'd wait until Zextras releases an updated Zimbradrive app.