To be clear the tweet says "The Garrosh heirlooms are specifically a reward for doing Flex+ while relevant. They won't be available once Draenor unlocks." This means they will still be in at 6.0.
Here is a link to the tweet:
https://mobile.twitter.com/WatcherDe...61202357592065 and
here is one to a battle.net forum discussion about it:
http://us.battle.net/wow/en/forum/topic/12675227507
To clarify, they will be dropping the main 6.0 patch before WoD. They have done that for every expansion so far and it doesn't seem like it will change. So you can still get them come 6.0 but not once WoD drops. That's what I understood the tweet to mean.