In what was one of the longest periods between content releases to date in Hearthstone, Blizzard introduced a new format called Standard. In it, old card sets cycle out, making room for new, different, and potentially less powerful cards to take their place.
This is important because it creates an environment where the developers can create cards that might be a bit on the strong side and not have to worry about them dominating the meta until the end of time. While the change seemed straightforward given the amount of time and clarity that was provided on Blizzard's behalf, some are still confused as to exactly how it will work.
For the record, Standard is currently:
For many it was easy enough when Blizzard simply told us that Curse of Naxxramas and Goblins vs Gnomes would no longer be playable. But what happens next year?
When the next cycle begins sometime in Spring 2017, during which a new year of competition will start, 2015's content will cycle out of Standard. There is no set date when this will happen. Whenever the first expansion releases in 2017 is when we'll lose access to 2015's cards.
This includes: Blackrock Mountain, The Grand Tournament, and The League of Explorers. Yes, it's hard to believe how shortlived the appearance of Reno Jackson is, but that's just how the current system works. Sets that release earlier in the year will naturally stick around for longer. Blizzard justifies this by saying they can release more risky content later in the year as it won't be troublesome for as long. But this also means we still have about a year and a half left with those fancy Old Gods.
As a reminder, you will no longer be able to purchase either of those two adventures or TGT packs. For the former, however, you can purchase a single wing of the content and retain access to buying the remaining wings at a later date. The latter will simply cease to exist. You can horde unopened packs on your account, but once they're opened, that's it. Blizzard has not announced any plans to rerelease old packs or create some sort of legacy pack; they have publicly said adventures could possibly return to the store though.
You will be able to obtain or get rid of cards from these sets through crafting, however. Just keep in mind that they'll only be playable in Wild, constructed Tavern Brawls (assuming the rules allow it), and future adventures.
Have unanswered questions? Head on over to the original announcement post for all sorts of details and insights from Blizzard.