I saw "Heizölrückstoßabdämpfung" as an example of a word that is almost a pangramm without repetitition, but is it a real German word?
It's a "real" word, in the sense that it doesn't contain any orthographical mistakes, but I seriously doubt that you'll find the word in technical literature.
My guess is that the word was composed for amusement's sake: getting the longest German isogram.
This word is made up only to be the longest "known" German isogram. So it is a phantasy term meaning really nothing. It doesn't make any sense, so I would not agree with the other answers stating that it is a "real" word.
If a 'heating oil backlash suppressor' existed, it would be a real noun. If it does not exist, it is not real, but still valid.
It is a constructed noun with no orthographical mistakes, and it is constructed according to the rules of german grammar and syntax. Because you can construct new nouns in german by putting two existing nouns (even constructed nouns) together, german vocabulary is at the same time finite and unbounded. :-)
Fun fact about german: You can combine almost any noun with any other and still have a valid noun, at least from a grammatical point of view. That doesn't always mean that the word itself makes sense or even represent an actual "thing" in the real world.
So to answer the question: Yes, it is. In this case, the word even describes something that actually exists.
Words like this are often referred to as "snake words" (Schlangewörter), because you can go on and add more nouns to it without making it "wrong".
Example: "Heizölrückstoßabdämpfungstechnikerkabinenschlüsseldekorationsbeauftragtenmobil" - Still a valid german word.
As indicated by markus, this is an example of compound word, a Kompositum. It is perfectly legal in German to create these from as many words as you like; the last word of the compound defines the main meaning, and the word or compound before that acts as refinement, not that different from english:
Kanne > Ölkanne > Heizölkanne (Can > oil can > heating oil can)
Whether the word still makes sense is another matter, of course. In theory, you could build an infinitely long word (and thus, if you do count compounds as "real words", German has infinite words).
So it depends on your definition whether Heizölrückstoßabdämpfung is a real word. Apparently, such an apparatus does not exist, but the meaning is clear, as Isotopp points out.
You could just invent such a thing, and you'll have invented a very real word as well :)
According to Wikipedia, Heizölrückstoßabdämpfung is fictitious.
Yet, there's the word Heizölrückstoßdämpfung meaning "heating oil rebound absorver". This word is both true in the sense it exists and it doesn't have grammatical mistakes.
As already written, this word once won the competition in 1998 about a german isogram with each letter only once appearing (german wikipedia), thus it is fictional.
The usage itself I cannot link as I have no student's account any longer to access the information. In my educational program it was widely used in programming courses to demonstrate how sorting algorithms and tree algorithms work.