Rainbow Six: Siege is five years old, and to celebrate Ubisoft is taking the game back to its roots. The time-limited Legacy Arcade mode runs until January 5, and reverts a game that has been tremendously well-supported and revamped over the years to roughly how it was in its first year. The maps go back to they way they were and looked then, the original 20 operators have their original loadouts, and while Tachanka players may not love the thought, everyone else will.
There is no question that Siege is a better game now than it was at launch. However, it suffers from the same problem of accrual that any game supported this well for this long has: there are a huge amount of operators and maps, meaning every time you go back there's more to learn. That's not necessarily a bad thing—it's just the way things are in the games-as-a-service world—but it does mean that the pure and focused core the game started with is buried under years of additional stuff. I played Siege at launch, and can't wait for the little nostalgia hit of playing that version of the game again (I'll have Sledge, thanks).
The mode does retain a couple of Siege's modern elements, but otherwise gets as close to those early days as it can. The update notes say "though your Operators will have their current abilities, all of your secondary gadgets will be fitting the original loadout of the game. That means no claymore, no impact or anything that was released later in the game. That also means that Ash, Bandit and Jager’s weapons will get back the scopes 2.5x(ACOG) they had upon release. The only exception being the deployable shields, which will remain the current ones."
As an additional festive treat, any player who takes part in the Legacy Arcade mode will receive one operator for free: and if you already have them all, you get a daft candy-cane Elf outfit for the character Ash.