I suggest that the engine is removed. To me it is too general and vague to serve a purpose.
I was going to ask about some details of questions tagged with engine -- What other tags do they feature? Is there a more appropriate tag that engine should be made a synonym of? Then I realized there's only one question so tagged. In that case I suggest you simply remove the existing tag by hand (you have more than enough reputation to do so). Yes, that will bump the respective question to the front page, but you shouldn't worry about one "reanimated" question.
If we decide to keep the tag, we should rename it to engines.
I think, the tag is useful for comparing the varios TeX engines. Questions which may deserve the tag engines:
- TeX Distribution and Engine
- The differences between TeX engines
- Differences between LuaTeX, ConTeXt and XeTeX
- Drawbacks of XeTeX/LuaTeX
Possible candidates are question which are about several engines, comparing, such as dealing with advantages and differences.
For example, this question doesn't deserve the engines tag, as it deals only with kerTeX:
It's the only one which has the engine tag, we should remove that tag there.
So my suggestion is: let's keep the tag, rename it, and sensibly retag some questions.