It's the most recent in a long series of Left 4 Dead 3 references found by the community.
It's become a cliche at this point: developer Valve seems to not want to release the third entries in its game series. Recently, however, a new reference to Left 4 Dead 3 was found in a Valve employee's development tutorial, rekindling speculation and hope for a third entry in the co-op zombie-killing series.
As noticed by fan site ValveTime, conceptual artist Tristan Reidford posted a picture in his tutorial on Destinations Workshop Tools of a file directory, and at the top of the list of folders is one titled "Left 4 Dead 3." The picture was quickly taken down and replaced with one without the suspiciously titled folder, but ValveTime managed to save the image.
Of course, this was possibly an accident and there's a very good chance that Valve employees all have Left 4 Dead 3, Half Life 3, and Portal 3 folders on their computers, regardless of the status of those games. However, it does seem a bit curious that the folder list is scrolled down so that the Left 4 Dead 3 folder is at the top. Also, as ValveTime points out, the new picture appears to be photoshopped.
This is the latest in a long list of references to Left 4 Dead 3 that've been uncovered, although it's the first recently. Back in 2013, Dota 2 fans touring Valve's office found a Left 4 Dead 3 listing. In 2014, the co-creator of Counter-Strike, who no longer works at Valve, stated that Left 4 Dead 3 was in development at that time and "looks great."