The thing with McCree's name is that unlike with real people referring to his 'deadname' isn't a violation of human decency, since he's, you know, not a real person. He's an entity whose traits are controlled by committee. Compare to buildings, e.g. Chicago has a skyscraper named Willis Tower, which used to be Sears Tower. Many still refer it with the old name, since that's what they've known it as and aren't obliged to show human decency towards it, since it's not a person.
With McCree you can also count in the matter of his name-change being highly questionable, so some may not be inclined to support it.
I've come to a compromise of headcanonizing Cassidy as his new code name, while McCree is still his real name (not that I ever refer to him as Cassidy, it's just what I think when someone else does).