BeatriceElysia: It weren't before. Now wallet founds are valid for one year.
Actually this has always been the case.
First, wallet funds are administrated per currency. If you use different currencies, be wary of Fair Price Package funds been unspent for a long time. (This is mostly an aside.)
Second, Fair Price Package (FPP) wallet funds have always had an expiry of one year from the last time your FPP balance was updated (by a game purchase adding FPP wallet funds). The expiry is refreshed any time you make a purchase that gives FPP funds.
Third, if you have purchased wallet funds directly, the full amount in your wallet doesn't expire, until you have spent all of your Wallet funds. Any FPP Wallet funds added by the last purchase which would reduce your balance to zero, will be subject to the one year expiry.
All of this was in the Wallet FAQ before the support pages got changed and the info is available here now:
https://support.gog.com/hc/en-us/articles/212819205-Do-GOG-Wallet-have-an-expiration-date-