Blizzard wants to recreate the content release similar to the original Vanilla WoW. They announced that the patch level will be at version 1.12 (all talents, items, class mechanics etc.) and content will be released in six phases. In a separate post they outlined when PvP content will be released. Combined this results in the following release plan:
This has an impact on what items are available/BiS at any given time.
There is a list by pingaan#5396
that is exhaustive for the majority of cases:
More TBD