The Silph Road has broken down what has been uncovered, with the main discovery being the presence of Pokédex data relating to second generation Pokémon. The version 0.45.0 APK code references #152 (Chikorita) to #251 (Celebi), although no character models or move set data is currently present.
Such Pokémon first appeared in Game Boy Color’s Pokémon Gold and Pokémon Silver, which later received Nintendo DS remakes in Pokémon HeartGold and Pokémon SoulSilver. It introduced Cyndaquil, Totodile, and Chikorita as starter Pokémon, as well as Ho-Oh and Lugia as Legendary Pokémon.
Niantic’s version 0.45.0 update notes only reveal daily bonuses, a momentary delay when defeating a Gym Leader at a rival Gym to let you place a Pokémon there, and Prestige adjustments.