I say burninate it.
If we need a plot-explanation tag, do we also need a motive-explanation tag (we actually have character-motivation, which has been used a grand total of 15 times, despite there being countless questions that are basically asking for character motivation; the irregular and inconsistent use is an argument against it being valid, imo)? How about species-identification? plot-mechanics? setting-explanation (note that this, too, exists as setting, and it has been used only 7 times)?
These, to me, represent some of the broadest categories of the most commonly-asked questions on the site. Most questions, imo, fall into one of those categories, or story-identification.
The few that I mentioned in that category are inconsistently and rarely used.
plot-explanation and plot are used far more often (about 100 times each), but I would argue that there are far more questions that they could be applied to than actually have the tag, which indicates that the tag isn't really effective.
Instead, I feel that they're being used as a fallback, when the OP doesn't know what else to use.
I say get rid of plot-explanation, character-motivation, plot, and setting.
They're all meta tags, and aren't doing anything productive as currently implemented.