This question is simple to ask, but complicated to answer 😂

When you post something on nostr, your app sends your post out to a number of different computers, called relays.

These relays forward your post to other user's apps, so people can read, reply, zap, etc.

So now, across the internet, there are a number of different relays and apps that all have their own copies of your post on their computers.

You can send a request to delete your post (basically sending another special kind of post) to these relays, but there are no guarantees that:

- these relays will delete your post

- these relays will forward that deletion request to the apps

- apps will delete your posts

- etc.

Since its so hard to guarantee something is deleted, most apps and relays today don’t bother trying.

With that said, storage is cheap but it isn’t free.

You also have no guarantee that your post will live forever.

It’s possible that within 5 years every app or relay we use today will shut down and get replaced by new apps and relays.

Things you post today can easily be lost in the shuffle.

TL;DR - It's a lot like the internet. If you write something, be prepared for the whole world to read it. If you want a guarantee that it lasts forever, make a backup.