The heirloom weapons from Garrosh (flex, normal and heroic) will still be available in 6.0 – the pre-expansion pack patch that will last around 4-6 weeks prior to the release of Warlords of Draenor. There are some changes planned for heirloom drops when 6.0 drops.
First, raid difficulties will change to the new structure, where Heroic becomes Mythic, Normal becomes Heroic and Flex becomes Normal.
Then, when a raider kills Garrosh on either Normal, Heroic or Mythic, they are guaranteed a 100% drop for a spec-appropriate heirloom weapon. The drop rate for subsequent heirlooms will also be increased, although we do not know by how much. However, for those after the guaranteed drop from the newly branded Mythic Garrosh (currently Heroic), keep in mind that it is also being reduced for a raid size of only 20, meaning there is much less room for carrying buyers and fewer of each guild’s raid group will be able to get in for those kills.
Once Warlords of Draenor launches, these Garrosh heirlooms will no longer be available, meaning many people won’t be able to complete a collection of heirlooms, regardless of which ilevel they are (Flex, Regular or Heroic).
For reference, here is MMO-Champion‘s handy heirloom list.
Heirloom drop rates are currently very low. I have one heirloom weapon on my main – thankfully a heroic one – despite over a dozen kills. Some guildies have 3+ while others have yet to see one drop. It is also worth noting that for Resto Shamans, the first heirloom for your spec is the staff, and not one of the Main Hand or Off Hand options, so chances are good that Shamans might not get the Intellect Mace or Shield, which is unfortunate for those looking for a bit more flexibility when they are replacing weapons at level 100.
They have also confirmed that the drop rate for the Kor’kron Juggernaut, the Heroic Garrosh mount, will change to a rare drop rate when Warlords of Draenor is released.
And for those looking for Ahead of the Curve or Cutting Edge Feats of Strength, they will be removed in 6.0.