Google's developer post "Gemini 1.5 Pro updates, 1.5 Flash debut and 2 new Gemma models" is dated 14 May 2024, the I/O keynote day, and says "introducing 1.5 Flash" today.[1]
At Google I/O 2024 Google debuts Gemini 1.5 Flash, a lighter and faster model with the same 1 million token context window as 1.5 Pro, alongside a 2 million token private preview for Pro.