Reached out to the dealer on putting the 821 low Termi on the 939 and what effect it might have on warranty. Here's the response.
"Howdy!
Any Ducati factory accessories that are installed on the bike don't void or nullify any warranty, however, the factory accessories must match the bike. All of the exhausts come with an upgraded air filter and a flash for the stock ECU to get the bikes fueling set up for the exhaust. As Ducati doesn't offer the low mount exhaust specifically for the 939 Hyper, it is very likely that putting it on your bike would void the warranty.
I'll ask our Ducati rep if he knows when/if they'll release the low mount for your bike though. I have a feeling it's in the works.
Hope this helps!"