For example, a set of transactions that occurred around July '09 where I consolidated 200 addresses into a 10k coin address for the "cool" factor when a friend of mine asked if it was possible to put that many coins into one address.
Something like that?
https://mempool.space/tx/e67c7cef9c59167046bee99a961a4ca75137c5ed4b697b30dc6e752ff1d50eccIn general, if you still have the same wallet, then you can sign messages from the old keys, even if coins are no longer there.
So, it is possible to for example sell coins in the past, and prove now, that you had them. But you should be careful, because even if it is "cool", then by correctly proving, that you had for example 10k coins, some people may want to attack you, if they will think, that you secretly still have a lot of money.