This proposal needs to put us at a good "resting place" both for Swift 5.3+ and Swift 6. The heuristic makes the forward scan work for enough existing APIs that I don't feel like we can simply drop it from Swift 6 without a suitable replacement. For example, I call out this SwiftUI example in the proposal:
func sheet(
isPresented: Binding<Bool>,
onDismiss: (() -> Void)? = nil,
content: @escaping () -> Content
) -> some View
The intent here is clear: the unlabeled trailing closure is supposed to be content, because the SwiftUI DSL is formulated that way. If we take away the heuristic but don't have some way to indicate SwiftUI's intent, we have a source break without a good path forward for developers. Of course, a proposal along the lines of what's mentioned in "Future directions" of the proposal could absolutely revoke the heuristic from some future Swift version (be it Swift 6 or later), but it needs to do so with full consideration of the source compatibility impact and the need to support API developers. We can't assume that such a proposal will come along and be accepted in time; SE-0286 has to leave the language in a good place for all of the language versions it touches (Swift 5.3+ and Swift 6+).
SwiftUI isn't an outlier here; I saw a number of examples when I did a source compatibility run with the forward+backward scan but without the heuristic (I had the same thought as you).
Doug