Many Nostr relays delete notes after a specific date. Keeping a local copy in Nostr clients I think is a good idea, in addition to paid relays with clear service expectations.
Discussion
Deleting data is a good practice in my eyes. The majority of notes become less valuable over time.
So why should we keep these notes on main relays especially when there relays operate altruistic.
When you want to save your notes and stuff the best thing is to run your own relay and backup the things from you and some specific people.
Saving things on the client side is a bad practice because over time you will lock yourself in to a specific client which has your data stored.
Running own relay is always good idea. But nostr is nowhere big enough to worry about long term note storage.
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.
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.