Via the CS:GO blog:

20TH ANNIVERSARY

• Celebrate 20 years of Counter-Strike with a retro version of Dust II, currently available in the Casual Dust II map group.
• The Chicken Capsule is now available for purchase in-game.

GAMEPLAY

• Adjustments to the AUG to bring its utility more in line with other rifles.
  • Slightly reduced rate of fire.
  • Reduced accuracy while unscoped.

DANGER ZONE

• Fixed an exploit involving duplication of items dropped by disarming another player.
• Players in warmup spawn closer to their teammate.
• Players in warmup now spawn with a random pistol.

Rumor has it:

• Believe it or not, retro Dust II isn't a separate map. It's instead made possible through the magic of different spawns (as explained by /u/Rock-C). Several users report that alternates between new and retro each round

  • Want to know what it looks like? Spoiler alert: It's the 1.6 version

• This is the Chicken Capsule, which contains various stickers with chickens (as the name implies) by a familiar community artist who also has another capsule in the game with their name on it, Slid3

• Chicken Capsule sticker images and Steam Community Market links are also up on CS:GO Stash

  • If you need full-res images, grab them here by pressing the Rich Diff button (the file icon) on the ones you want: https://github.com/SteamDatabase/GameTracking-CSGO/commit/8c5ea870d7cf0a35c2c94bf98b70e9e194ca0d06

• Size is close to 75 MB