Does anyone see reasons for or against the
caustics tag including prism topics?
Caustics are not explicitly prism-rendering, but caustics are often prismatic in small degree.
Or, could we create a
prism tag for following related topic?
While they both are about rendering light based effects, that is where the similarity ends.
No I would not include prisms in the caustics tag, nor synonymize them.
If you feel we need a tag for prismatic effects, then I'd normally say just create a new prism tag (even though I think that is is rather ambiguous), however seeing the very low usage of the caustics tag how about we rename it to something like "rendered optical effects." That would include both prisms, caustics, and other effects like refraction.