I don’t know much about cryptography, but there’s probably a way to craft an arbitrarily weak private key that wouldn’t take much effort to break. This key could then be used to sign “ephemeral events.” If we want to mimic an ephemeral message lasting X seconds, the ephemeral key should have a corresponding level of difficulty to break within X seconds (not necessarily linear; this is just a simplistic example). If the message should "last" 10X seconds, the ephemeral private key should be 10X weaker, and so on.

In many cases, denying authorship is more critical than simply deleting an event.

The good thing about this approach is that, after sending an ephemeral message (that may have a timestamp), as time passes, it becomes increasingly harder to assert with confidence that I was the original author.