When did chess engines first assign a high value to a pawn on the sixth rank, with an opposing pawn in front of it? (This pawn is called a "thorn pawn", "fawn pawn", or "nail pawn".)
I would assume such pawns have always be valued somewhat higher (generally) by chess engines since they were around. Doing so follows from rather basic heuristics, that weak engines long ago could have followed:
1) Controlling important squares. Taking the example of the e6-pawn (for the points below as well), it controls the f7- and d7-squares.
2) Potentially blocking in enemy pieces. If Black's bishop were still on f8 then it would have to come out via ...g6 and ...Bg7. Black's major pieces are also prevented from fighting for the e-file.
3) Conversely, White's major pieces have free reign over the e-file.
Of course, sometimes a pawn on the 6th rank could prove to be a weakness. But if it's protected enough, usually the benefits are clear.