A thread has been posted on Neogaf by user "CBOAT" (crazy buttocks on a train) showcasing the above confidential PowerPoint presentation from Valve. As CBOAT has a reputation for leaking legitimate information there is a very high chance these are genuine.
The presentation features images showing off the company's next-gen gaming engine, Source 2. The images use an updated Left 4 Dead 2 prototype to show the advances in technology.
One slide features an updated version of the the Plantation level from Left 4 Dead 2. It shows how there is a significant increase in detail using the Source 2 engine. Lighting and shadow effects are also improved.
It is definitely interesting to get a look at Valve's new gaming engine, despite the fact rumours abound that these slides are in fact 3 years old.