The Left 4 Dead/Payday project that Valve and Overkill have been planning is not a sequel, though it’s easy to understand why rumors would have labeled it as such.
It is in fact a new mission for Payday taking place at No Mercy hospital from Left 4 Dead lore. It was one of the first locations the outbreak happened, though that’s as far as it goes.
Valve’s Chet Faliszek explains: “This is a 100% Overkill made mission for Payday: Heist simply set in no mercy hospital from the Left 4 Dead series. It is not telling the origins of the Infection and is not canon. There is a little cameo from one of the L4D characters (the comic should give you a hint) and a fun little easter egg with a payoff later in Left 4 Dead. Just us having some fun with a studio we are friends with and who have also made a great co-op game.”
No dates on when the mission might release just yet.