An image model went GA and its predecessor was deprecated in the same changelog entry
Google's 6 October changelog entry marks Gemini Nano Banana 2.1 as generally available and, in the same breath, records that the gemini-3.1-flash-image model has been retired. Same product family, same day, no transition period worth mentioning.
Image generation is the layer where model swaps are most visible to end users, because style changes. A tool that advertises "the best image model" is advertising a moving part, and when the vendor retires it the tool either upgrades quietly or quietly gets worse. Ask which one happened last time you noticed a change in output.