Zimbra 8.8.15 has been released last week. According to Zimbra product lifecycle, this release will be an LTS (Long Term Support) release and will have general support until 2022 and tehnical guidance until 2023.
As Zimbra version 8.8.12 general support will be ended on September 2019, I would prefer to upgrade all of our Zimbra deployment before the date and our current deployment (internal use) as our initial test.
We are currently using Zimbra NE 8.8.12 patch 4 and run on top of cloud system with almost all feature installed. Running on top of cloud infrastructure has a flexible advantage, something like easier backup as precaution before starting upgrade procedure.
Our upgrade process works out as expected and took a few minutes from downloading upgrade package, execute ./install.sh and restarting services to make sure everything goes smoothly.
Please refer to Zimbra 8.8.15 wiki for fixed issue and known bug and release notes.