The more I watch it, the only player who Rashford in fact has any impact on is Ederson. But nobody forces Ederson to come out like that, that's just dumb decision making. It's not offside at all. Akanji is taken out by the pass and is multiple yards away from the ball at all times. Rashford has 0 impact on him, it just goes past him. Walker is covering Bruno but from the wrong side so again, Rashford has 0 influence there. It's only Ederson who could argue Rashford interfered, but either way him and Bruno were in the same position so it didn't make any difference. That's a definite goal the more I look at it.
By the rules, of course it's a valid goal. But even in the spirit of offside, you're never going to change the rules to make that offside once you analyze it deeper. City defenders are all far away and Ederson just makes a dumb decision.