I think there are a couple of reasons why he is scapegoated by England fans.
One is that over the past few years he's been England's main (and only) real creative threat. Not having the talent around him that he has at City can see him trying to do too much, not trusting those around him and and often losing possession. He was singled out at Euro 2016 but there were players who performed much worse than him that got off lightly.
Second would be the media narrative that surrounded him when he moved to City, that has certainly had influence over how people perceive him.
You can give credit to Southgate for sticking with him, but I wouldn't say he's proved anyone wrong just yet. His performances, as with the whole team, have been pretty poor (to be fair his goals are a pretty f**king important). I'd still start Sterling as I belive he is a quality player and that hios form will turn around plus Rashford, Saka and Sancho are not as good as him, but shift him to the right to allow Foden to play in his preferred position.
The more think about it the more I see us a favourites for this game. I just hope the occasion and pressure doesn't get to them, this could be Southgate's strength as a coach.