If the simulation was quantized, say like using integers of a large enough size to encompass all things in your simulation, it would be fully reversible I believe. Of course I'm speaking from my limited understanding of physics engines in computers today like bullet or equivalent. Isn't floating point just like a compression for large numbers in limited bits, and the only reason it would not be reversible? The other potential non-reversibility is the fact that the physics engine uses some sort of integration like adding up forces at each time step unlike the universe which I believe is instant. (verlet integration? Google says that is reversible.)