The metallic plate behind >2010's CPU's heatspreading plate is the actual chip die substrate?
In most if not all modern processors, the silicon is flip-chip bonded onto an interposer which then has all the connection pads on it. As a result the back of the silicon die is then at the top - pointing to where the heatsink is attached.

In desktop processors, this is then typically bonded with thermal compound to the top metal shell, thus allowing good heat transfer from the die to the heatsink. In fact this is why with some of the very new processors you have to be careful how tightly you screw on heatsinks as it is possible to quite literally fracture the silicon if the metal shell gets deformed from pressure. The result is something like this: Image Source



For laptop CPUs, a similar process is used, except the metal shell is omitted to save space and weight. The heatsink in this case attaches directly to the silicon die. Generally thermal pads or at least a thick layer of thermal compound are used to avoid chipping or cracking the silicon when the heatsink is attached. The result is like this: Image Source



The same process is used in many other applications. TO-220 packages as you mentioned have the wafer directly bonded to the back metal pad and then the pins are wire-bonded to the front. Large FPGAs that run at high speeds use a similar package to desktop CPUs - flip-chip to an interposer with a metal top shell.



To further answer the point of finding formal resources, there is probably none more formal than the Intel Packaging Databook which while primarily seems to describe various mechanical dimensions, it does also in the introduction and packaging materials section go into the flip-chip BGA package structure. It also mentions (which relates to the non-lidded version) that:


  The die backside is exposed allowing the thermal solutions and thermal interface material to have direct contact with the die surface.


I did try to see if I could find what exactly is done to the back of the die for protection, but there isn't anything specifically mentioned. In all likelihood it will essentially be nothing more than a passivation layer - typically Silicon Nitride or Silicon Carbide.