In a sense, data ownership on traditional platforms like Facebook and Twitter is governed by a terms of service, but on Nostr, it feels somewhat closer to publishing to the public domain. 
It doesn't feel forthcoming to compare Twitter's ability to delete your data upon request to Nostr's. Someone could, theoretically, be backing up your tweets, sure. But large platforms do quite a bit to prevent that, it's against their terms, and anyone using it would or should know that the data was obtained in bad faith. On Nostr that's not the case. Nostr's operating model has no real agreements. And the core idea of the model is that anyone can and should be backing up that data on their own servers. It seems much more reasonable for someone to think that the data could be used for whatever they want in that model. That is the de facto ownership model on Nostr and fediverse content right now. You own the identity, but you don't own your data.

To be clear -- I don't think that's bad. I *like* the idea of that data being free and open when it's published -- who owns your words after you say them aloud?
But it does inform what data I put here. And I think that's a big shift for most people coming from large traditional platforms. We should embrace that distinction to help people's experience here. 

 > In a sense, data ownership on traditional platforms like Facebook and Twitter is governed by a terms of service, but on Nostr, it feels somewhat closer to publishing to the public domain. 

TOS are a contract. They only mean something because of the law.
Copyright law and privacy law still apply in the absence of a TOS document.
Of course relays and other parties can break the law, but the same would apply for mainstream platforms.

> Someone could, theoretically, be backing up your tweets, sure.

It's not theoretical, it's very doable.
This has been done before.

>  But large platforms do quite a bit to prevent that,

They can't prevent that. It's why there was an archive of Trump's tweets, for example.
Also tweets end up all the time on the Wayback Machine by the Internet Archive. I have accessed deleted tweets before this way.
In fact, backups of large portions of social media platforms have been made before by data hoarders.

> it's against their terms

People can break TOS.

> who owns your words after you say them aloud?

If they are copyright-worthy and recorded, the speaker owns them.

Mainstream platforms sometimes give users the delusion that they will be able to take things back, but they won't, or at least there is no guarantee that they will.