I am curious about it. I wonder if some people aren't distinguishing between the perfect and the pluperfect, or it might just be a misunderstanding. I can't think of any reason why the pluperfect would fall out of use. I is useful and different from the other tenses. I can't imagine it being regional.
Don't get too confused about it. "Pluperfect" and "past perfect" refer to the same construction. The terms have a different theoretic context, but that's not too important for writers.
Pluperfect is a latinate term. There is a tense (an inflection of the verb) in latin called the "plus quam perfectum" (or similar), which morphed into "pluperfect" in English (and "Plusquamperfect" in German, for example). Traditional grammar treats it as a tense of its own.
Most recent grammars distinquish between tense and aspect: It's past tense, and perfect aspect (for short "past perfect" and you can add the term "tense" to that that).
The aspects are: simple, perfect and continuous.
The "imperfect aspect" you mentioned does exist, although it's a different distinction: "perfective vs. imperfective aspect". The perfective aspect is roughly equivalent with the simple aspect, and the imperfective aspect is roughly equivalent to the continuous aspect. (I say roughly, because theories never quite match.)
To confuse matters further, there is an "imperfect tense", which is latinate, too. In English that would be "past continuous" or "past imperfective", but in Latin it's a tense of its own.
Using the latinate terms like this is sort of controversial, though the fit, as far as I understand it - I'm not really that qualified, is better for the pluperfect than the imperfect.
In the end, it doesn't matter much which terms you use, as long as your understood. "Pluperfect" is less common than "past perfect", but it's still not obscure.
I've never heard "superperfect" and a superficial google search only gives me this thread.
