Is USB host always protected from reverse current?
In principle could one assume that all desktops and laptops are not
  damaged by a reverse current?


No, you cannot assume this, especially in such a generic form. Some (high-end) desktops might have high-side switches to control VBUS and limit output current, but most do not have this electronics, and use fuses. Fuses do not protect from reverse currents. Some hi-side switches do have reverse current protection, some don't.

Again, the result may depend on the level of applied voltage. If you apply a 5-V (+-5%) source to a VBUS on a standard USB port, it is likely that the voltage difference will be "absorbed" over resistive elements (wires-traces-fuses), and just an overcurrent of some sort might occur, with no damage. But if you apply a 19-V 4.5-A power brick to VBUS on a standard Type-A port, I am sure some bad things will happen to your PC. 

You won't find any "technical documents" on reverse currents because the entire USB design prevents "accidental" reverse flow on USB ports. In legacy USB (Type-A Type-B connectors) this protection was achieved by mandatory cables, which were only A to B, and not anything else. In advanced USB with symmetrical Type-C connectors it is achieved with a requirement of "no VBUS active ever" until sides determine their power roles over CC-interface.