Except..mandatory means not mandatory in Ledger-language.
Shout out to @bengy for telling me about this important news. Thanks a lot man!
Why Update
Some security researchers have discovered a few security vulnerabilities in the Nano S. Ledger themselves say that these vulnerabilities are serious, but not critical. Meaning that potential attacks/hacks are possible, but very impractical - and has not ever happened. They can't share details now, but a full blog post will explain on March 20th.
The update of your Ledger Nano S to the firmware 1.4.x is strongly recommended, and will become mandatory on March 20th. Until this date, you are free to use either firmware 1.3.1 or firmware 1.4.x. By mandatory we mean that the Ledger Wallet application will bug you with a popup asking to update your device (but it won't be blocking)
Here is the FAQ
Here is the GUIDE
https://support.ledgerwallet.com/hc/en-us/articles/360001340473