The purpose of the new series is just to keep up with counterfeit money, to include newer security measures.
But yes, countries in the world have revoked currencies from time to time.
I'm trying to search for this, but I can't find anything about a 20 year validity.
Here for instances the information if the German Bundesbank, where you can still swap with the official rate of 1.95583 DM/€. https://www.bundesbank.de/de/aufgaben/bargeld/dm-banknoten-u... (Currently there is a restriction of in person service due to corona, but if you trust mail it works, else you have to wait for that Corona restriction to be lifted)
That exchange works without limits or anything.
This total sum seemed to be far beyond the robbers' actual intentions, and the Northern Bank quickly issued a new banknote design, offering free exchanges for anyone with the old one.
Of course the robbed money had serial numbers, so it's now generally believed the IRA just burned most of the stolen cash.
"Note that it is U.S. government policy that all designs of Federal Reserve notes remain legal tender, or legally valid for payments, regardless of when they were issued. This policy includes all denominations of Federal Reserve notes, from 1914 to present as per 31 U.S.C. § 5103." https://www.uscurrency.gov/acceptance-and-use-older-design-f...
Despite not being legal tender anymore, the Deutsche Bundesbank changes your DM notes, if they are after issued after 1948 (https://www.bundesbank.de/en/tasks/cash-management/dm-bankno...). The currency has been introduced in 1948.
If you feel inclined to do so, you can spend a 1 Yen note from 1885 https://en.wikipedia.org/wiki/Banknotes_of_the_Japanese_yen
So, inflation and wear and tear takes old notes out of circulation rather than expiry.