You're not really missing anything here - it's a very focused API endpoint (that I believe was meant to support some of the internals/optimizations of SwiftUI - but I'm asserting that with no detailed knowledge).
the objectWillChange publisher's Output type is type-aliased to Void, so while you'll get a triggered notification, it's also not actually passing any changed values - just the notification that the referenced object was, in fact, changed (or is about to change) in some fashion - so that your code can open up that object and inspect it for what it is after the change.