Just before all the hubbub about the Sims 4 anniversary updates yesterday, its predecessor got an unexpected update too. The Sims 3 just got its first patch in years this week and so far EA's just said nothing about it.
According to an old archive of Sims 3 patch notes, the last time the game was updated was in January 2014. There's no new entry for this week's patch to explain what's up, though one helpful player in the EA Answers Forum explains that the patch addresses an old issue with the game failing to launch on newer Intel processors.
This seems to be a patch only related to the EA App version of the game and doesn't have a corresponding Steam update entry by the looks of it on SteamDB.
After the update you may get a new popup between hitting play in the launcher and the game opening which asks you to activate your product.
The same helpful simmer suggests clicking «exit» which will allow you to continue. I entered my EA App credentials and was waved through that way without issue.