An .IPA file is the iOS application archive format used to install apps on Apple devices. Version 1.6.3 of the original Angry Birds game (released around mid-2011) is considered by many preservationists and fans to be the definitive "classic era" build before massive user interface overhauls and power-up systems changed the core gameplay loop. 🕹️ Why Version 1.6.3 Matters
Why do purists hunt for the 1.6.x IPAs specifically? Because of the physics engine.
Before Rovio moved to a more polished, vector-art style for Angry Birds 2 and their movies, the original game had a textured, almost hand-drawn charm.