Famed NeoGAF leaker CBOAT has posted a screencap of what appears to be a Valve slideshow presentation detailing Source 2. The PowerPoint shows Left 4 Dead 2's plantation level rebuilt in the new engine and one of the slides, shown above, is clearly visible. Compressed, but visible.
I don't believe this necessarily backs up Left 4 Dead 3 as something which exists, however that is likely a game we will one day get to play. The leak is the first we've really seen of Source 2 in quite a while, though, and I'm ready to hear more from Valve -- whether that be about the engine, the next game, or probably both.
l4d3? (Source 2 reveal) [NeoGAF]
For reference, here's the plantation in Source: