Heirlooms have never dropped in LFR, and that's not changing in 6.0. "Normal" (new Flex) will drop them, however, and anyone within spitting distance of ilevel 540 shouldn't have a hard time getting into or creating a Normal raid.
I'm not saying your wrong but what I'm reading on mmo-champ makes it look like heirlooms drop from any difficulty.
Siege of Orgrimmar Changes
Siege of Orgrimmar is giving out Heirloom weapons and has been converted to the new Warlords of Draenor raid size system.
Your first heirloom weapon drop per difficulty is now guaranteed. These weapons will scale up while leveling and won't be replaced until at least level 100, so be sure to grab some for yourself and alts!
The Warlords of Draenor raid size system will be applied to Siege of Orgrimmar:
There are now four difficulties: Raid Finder, Normal (was Flex), Heroic (was Normal), and Mythic (was Heroic).
Normal and Heroic will scale from 10 to 30 players. Raid Finder can also scale down if players drop group.
Mythic will be a fixed size of 20 players.
Each difficulty has its own weekly lockout.
Raid Finder, Normal, and Heroic have a loot based lockout, which means you can kill bosses as many times as you would like, but you will only get loot in each difficulty once a week. The amount of loot that drops scales to the number of people eligible for loot, so you can't game the system.
Mythic difficulty has an ID based lockout, similar to how Heroic lockouts have worked previously. You are locked to one raid ID for the week.