The introduction of the Quantum Extremal Surfaces (QES) and the Generalized Second Law (GSL) of blackhole thermodynamics paved the way for the derivation of the Page curve.
This was done in parallel by Almheiri-Engelhardtt-Marolf-Maxfield as well as Penington in 2019.
Page curve is the curve followed by blackhole entropy and in deriving this we get the result that quantum information is not lost in the process of blackhole evaporation.