The holographic principle in theoretical physics was derived from black hole thermodynamics and it basically says that contrary to what we would usually say, entropy is not really proportional to the volume but rather the surface area.
However, are there any generalizations of holography that would apply to models of physics where entropy would indeed be proportional to the volume?
A model of physics with entropy proportional to the volume just sounds like any QFT in a fixed background. As discussed in various papers, you could put this QFT in a fixed AdS background and then its S-matrix will be described by a nonlocal CFT on the boundary.
The price to pay is that you have to take the flat-space limit for this to work. And this requires a lot of information about the CFT... operators with arbitrarily high scaling dimensions no matter how light the scattering particles are. And it's important to note that the bulk and the boundary are allowed to exchange energy in this setup which means it is not really holography. I.e. if you don't take the flat-space limit, then there is no sense in which these boundary correlators encode complete information about the bulk. There are genuine AdS observables that they miss. But of course there have to be when one side has $L^d$ degrees of freedom and the other has $L^{d + 1}$.
