Look at the google quick answer box for both "caustics" and "prism."
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 ...
The renaming of the render engine tags is mainly related to reducing the misuse of the rather generically named blender-render tag. Whether all the other render engine tags need renaming because of this, I don't know. I'm certainly not against having all the tags be consistent, but if there's no ambiguity with the other tags, then maybe they ...
It appears that was the reason. There was a relationship between preferences and user-preferences it seems. I went ahead and made both preferences and user-preferences a synonym of customization and merged all previous questions into that.
Synonyms exist so that commonly used words which are incorrect can be remapped to the correct tag that should be used. Generally, our agreed upon policy is if the tag falls under a broader one, merge/synonimize it.
The tags you mentioned have been been in the waiting list for sometime now. They have been removed.