I don't think it's bad practice for clients to keep a local copy because it provides an opportunity to move to a paid or self-hosted relay after finding more value in it. Alternatively, there can be alternative means of redundancy.
Discussion
The problem I see is how do you get the data from a client back to a relay.
But I think we are talking about different things. 🤔
You said a local copy. Which is fine in my eyes. But a client app or website like damus or iris shouldn’t backup your history in local app or browser storage.