Distributed Ray-Tracing

Straw man.

> Shadows have a hard edge, as only infinitesimally small point light sources of zero volume can be simulated

Uh, no. Raytracing can definitely have emitting surfaces and volumes.

> Reflection / Refraction can only simulate a limited set of light paths, for perfect mirror surfaces, or perfectly homogeneous transparent media.

You sure about that?

> More complex effects like depth of field are not supported.

https://www.povray.org/documentation/view/3.60/248/

Also, the title should get a "2019" tag.

The simplified history is usually presented as Whitted Raytracing -> Distributed Raytracing -> Path Tracing.

The gist is that in Whitted for each surface hit a single shadow ray per light, a reflection ray and a refraction ray are traced. Shadows and reflections are perfectly hard. Distributed raytracing takes all those single rays and shoots N randomized rays instead, which gives soft reflections and shadows. Neither of these orthodox algorithms imply indirect lighting, which is what Path Tracing added into the mix.

This is not considering other light transport algorithms such as radiosity or photon mapping, which were popular ways of doing more cost effective global illumination in the nineties and noughties.