Why does ShareSecret expire secrets after a period of time?
The best way to keep sensitive data safe is to have no data at all. One of the things that makes sharing sensitive information so dangerous is that the methods by which data is typically shared -- email, SMS, group chat -- will store your data forever. Or at least until you manually delete it or remove it from those platforms.
For example, if your accountant needs your social security for your tax return, and they ask for that information in an email, they only need to access that information once. There is no reason for your social security number to sit around in your sent emails forever, and doing so is very risky!