Disagree, cultural norms are not just formless things that can come and go entirely without context. We can never have a discussion about the morality of slavery without the historical context.

You aren't disagreeing with me, you're agreeing with me. You're saying that ethical and moral claims are context dependent, not absolute. Such claims include claims that, for example, abolishing slavery or lynching is an "improvement". Which is exactly my point.

I don't think it's safe to assume something obviously bad today won't be good by many/most measures tomorrow. In fact I'm quite certain that the advancement of society doesn't rely exclusively on constraints, but also on expanding freedoms and loosing poorly conceived constraints.