When C# shipped this change I was working on a large C# codebase and we were very worried about this. After auditing the entire codebase and checking every for

When C# shipped this change I was working on a large C# codebase and we were very worried about this. After auditing the entire codebase and checking every for loop, we found two pre-existing bugs in code which expected the 5.0 behavior and zero places which relied on the old behavior. Everyone that I've ever talked to who went through the transition has a similar story. In practice no one even incidentally relied on the old behavior. Go rolling it out as an experimental opt-in feature is a good idea, but it'll probably turn out to be unnecessarily cautious and they could have gotten away with just unconditionally enabling it.