The pawn on d6 in this position is never going to be all right. Its neighbour on the c-file was traded off long ago, its other neighbour has already run past it to e5, and the square in front is covered by the white pawn on e4. So it cannot be defended by a pawn, and it cannot advance.
7 
2 
d 
g Three conditions, all at once
Behind its neighbours, undefendable by a pawn, and blocked from advancing. Take away any one of those and it is not a backward pawn. A pawn on d6 with a pawn still on c7 is fine, because …c6 and …d5 or simply …c5 will sort it out in two moves. A pawn on d6 with nothing controlling d5 is also fine, because it pushes and disappears.
What makes it permanent is the enemy pawn or piece sitting on the square in front. In the diagram the pawn on e4 does that job for free, forever, without moving.
It always lives on a half-open file
Notice that the same missing c-pawn does two jobs for White. It means d6 will never be defended, and it means the d-file has no black pawn on it, so White’s rook on d1 attacks the pawn without anything in the way. That is a half-open file with a target at the end of it, and it is the whole reason backward pawns matter: a weakness you cannot reach is not a weakness.
And there is a second gift on the same square. d5 is not just where the pawn cannot go, it is a hole that no black pawn can ever cover, which makes it an outpost for a white knight. Weak pawn, weak square, one file, both permanent.
Curing one
Three ways, in the order I would try them. Push it. Getting …d5 in, even at the cost of a pawn, kills the weakness dead, and the defender should be watching for that break from the moment the pawn goes backward. Trade it: swap it off against an enemy pawn, or swap the pieces that attack it until nobody is left to pile up. Or ignore it and play elsewhere, which is what strong players usually do, because a pawn that needs three enemy pieces to attack it is buying you a free hand on the other wing.
What you must not do is defend it passively with rooks and wait. That is how you end up in a rook ending a pawn down, defending the same pawn with no pieces left to trade.