Some are. It depends on the kind number.

Each event (note) has a kind number. Kind=1 notes are regular text like this message to you. Kind=7 is a reaction. Kind=5 is a deletion, and so on.

Every event has a unique ID, and so they can’t be replaced by ID alone. Replaceable events have kind numbers in specific ranges. Based on the author’s npub, kind number, and tags (parameters) some events can be replaced by later events.

So for example, if you write a long-form article and want to be able to update it, you could publish it as a parameterized, replaceable event so that later if you want to make edits, relays will accept your replacement event.