We can only measure a black hole's mass, electrical charge, and angular momentum.
But, according to the classical Einstein equations, black holes are disturbingly simple; their only properties are mass, electrical charge and angular momentum.
But inside a black hole, only properties like mass, charge, and angular momentum matter.
The only properties we know to assign to a black hole are very straightforward: mass, charge, and angular momentum.
Instead, all that will remain, information-wise, is what the total mass, charge, and angular momentum of the black hole now is.
Yet, whatever falls in contains information, while the black hole itself, at least in General Relativity, is defined only by its mass, charge, and angular momentum.
All the details of the matter are completely lost; all that is left is the geometrical properties of the black hole which can be identified with mass, charge, and angular momentum.
No matter what it is that you make your black hole out of – whether it's stars, atoms, protons, electrons, antimatter, heavy elements, or exotic particles – there are only three things that matter for the properties a black hole possesses: its total mass, electric charge, and angular momentum.
While the mass of a black hole can take any positive value, the charge and angular momentum are constrained by the mass.
Regardless of the type of matter which goes into a black hole, it appears that only information concerning the total mass, charge, and angular momentum are conserved.
