BindableObjectのdidChangeは毎回同じものを返してね

ContentView.swift

import Combine
import SwiftUI

class MyPublisher<T, E>: Publisher where E: Error {
    func receive<S>(subscriber: S) where S : Subscriber, Failure == S.Failure, Output == S.Input {
        inner.receive(subscriber: subscriber)
        Swift.print(#function, #line)
    }

    typealias Output = T
    typealias Failure = E

    var inner: AnyPublisher<T, E>

    init(_ inner: AnyPublisher<T, E>) {
        self.inner = inner
        Swift.print(#function, #line)
    }

    deinit {
        Swift.print(#function, #line)
    }
}

class ContentViewModel: BindableObject {
    // ↓こちらだと動く
//    let didChange = PassthroughSubject<ContentViewModel, Never>()

    // ↓こちらはselectionなどで別途更新をかけないと動かない
//    lazy var didChange: MyPublisher<ContentViewModel, Never> = MyPublisher(didChangeSubject.eraseToAnyPublisher())

    var didChange: MyPublisher<ContentViewModel, Never> {
        MyPublisher(didChangeSubject.eraseToAnyPublisher())
    }

    private let didChangeSubject = PassthroughSubject<ContentViewModel, Never>()

    var objects: [String] {
        (0...100).map(String.init)
    }

    var selected: String? {
        didSet {
//            didChange.send(self)
            didChangeSubject.send(self)
        }
    }
}

struct ContentView : View {
    @ObjectBinding var viewModel: ContentViewModel

    var body: some View {
        List(viewModel.objects.identified(by: \.self), action: { o in
            self.viewModel.selected = o
        }) { o in
            Text(o)
                .background(self.viewModel.selected == o ? Color.red : Color.white)
        }
    }
}

#if DEBUG
struct ContentView_Previews : PreviewProvider {
    static var previews: some View {
        ContentView(viewModel: ContentViewModel())
    }
}
#endif