In the Implementation Guide (IMG) there is a new IMG node under General Settings -> Currencies:
Using this function you can allow currencies to expire. This means that
you can attach warning or error messages to the creation/change of
objects in these currencies. You can use this function, for example,
for the currencies participating in the European Monetary Union, that
then lose their validity after the end of the dual currency phase. You
can thus prevent objects in these expiring currencies being created or changed.
A further application of this function is for the expiry of currencies in high inflation countries.