This was done by Paul Graham himself in this forum's early days.

At that time reddit was rising. PG didn't want to turn this forum one into another Reddit. To discourage posts with body he grayed out the body text. And then within the next few weeks he grayed out the texts gradually even more. Until it reached the point that you see now.

Encouragement was to post interesting links and blogs. But not turn this one into a general discussion forum.

That was may be in the late 2000s. As far as I can recall.