Since 2012 R2 is Windows 8.1-based, I’m assuming it will be out of support as well, despite Microsoft still supporting it for several more months. The forum announcement mentions version 1607, which corresponds to Windows 10 RS1/Server 2016.
Yes, you can stick to 1.31 and everything will still work (make sure to turn off automatic updates in your server settings), there are just no compatibility guarantees from 1.32 onwards. The one thing to keep an eye on is any security-related announcements, since being stuck on 1.31 means no security updates, unless Plex decides something is important enough to backport to 1.31.
