Is he really City academy product? They signed him from Watford when he was 15 and left them when he was 17. He spent just 2 years at City.
Trippier, now he is a proper City academy product.
Tbh the rivalry between Utd and City has generally tended to be a one-way situation. With them hating us, and us not generally being that arsed about them. Even now, I'd say it's the same. Right now, I'd put City in the same tier that Arsenal were under Wenger (when they were good). Strong (though not necessarily as fierce as even Arsenal were back then) sporting rivalry, but nothing deeper than that.
Put it this way, we wouldn't boo a random former Arsenal player like Iwobi if he came to OT with his present team. And we generally would do the same with a random former City player as well, by and large.
Liverpool, and maybe Leeds on the other hand, are a different matter.
Re Sancho, he spent 2 and a bit years at City, and by all accounts had strong (maybe even stronger) friendships with his age mates at Utd like Gomes and Rashford. I don't think there's an issue as far as we're concerned. City fans might hate him as a point of principle, but even that might be doubtful considering KDB and Salah haven't exactly set pulses racing at Stamford Bridge despite making them look like bigger fools than what Sancho has done thus far. Plus, it's not like they're missing him in any way.