According to Blizzard, “In order to keep Hearthstone exciting and accessible as more cards are added, Standard allows players to use the most recently released cards, as well as a core of Basic and Classic cards. Our intention with Standard is to keep the game feeling fresh for all of our players, and to allow newer players to jump into Hearthstone quickly, without the need to collect lots of cards to build a competitive deck. Standard also gives our developers more freedom when designing new cards for the future.”
To that end, when the next Hearthstone expansion is released in the next couple of months, Blizzard will phase out sets that weren’t released in the same year or the year prior, meaning that those cards will no longer be considered as part of the Standard format. Instead they will move to the “Wild” format.
The Standard format moving forwards, for the time-being, will include cards from Basic, Classic, Journey to Un’Goro, Knights of the Frozen Throne, Kobolds & Catacombs, and the upcoming 2018 expansion. The Wild format will also be getting older expansions such as Whispers of the Old Gods, One Night in Karazhan, and Mean Streets of Gadgetzan.