Altcoin where coins become unusable after some time? (so you have to use them)
Are there any?
i dont really see a point for this, as you can always send coins to different addresses you have control of.
The coins would be traced to the block they were created in. If that block is e.g. "1000 blocks old" the coins are invalid.
so you would effectively create a self destructing money supply with the last one who is holding the coins to loose them?
sorry, i dont really get what you want to achieve with it.