My first job in journalism was to write obituaries for the Colorado Springs Sun, which in 1986 became the subject of an obituary itself. There I was taught not to use the phrase "passed away." "It is a euphemism for people who are uncomfortable with 'death.' And people who aren't comfortable with 'death,' don't read the obituary section," I was told.
The phrase "was dead" is, of course, written in the passive voice, the use of which causes the hair to rise on the arms of lovers of Strunk and White's "The Elements of Style." [This assumes of course, that other assaults on linguistic purity has not already made the book's lovers bald from frustration. I digress.] On pages 18-19 Strunk and White compare two sentences: (1) "There were a great number of dead leaves covering the ground," and (2) "Dead leaves covered the ground." The second choice is in the active voice, which is more often than not better because, according to the authors, (a) the "habitual use of the active voice makes writing more foreceful," and (b) use of the active voice usually makes the sentence shorter.
In my experience since leaving journalism and going to law school -- where any talent for writing I might have had was brutally beaten out of me -- some lawyers prefer the passive voice to establish tonal consistency; they reserve the active voice for when they want to sound forceful, as if their stationary wasn't already designed to accomplish that goal. Legal writing critics, like Bryan Garner, encourage the habitual use of the active voice which will "largely prevent convoluted, backward-sounding sentences in your writing."
But even more so, lawyers (and others) like the passive voice because they think it avoids a conclusion of responsibility. The argument goes like this: "Saying that 'the cat died" rather than 'the cat was dead' implies someone killed the cat!" NO IT DOESN'T! But hint hint -- if you think someone is trying to cover-up something, especially their own responsibility, I'll bet their statements were in the passive voice.
C. Edward Good's "Mightier Than The Sword," pp 124-126 (Blue Jean Press 1989) gives a good discussion on that thought.
The sentence you pose, however, is not easily put into the active voice. But I think the active voice would work if we said: "Please confirm that the cat died." Or "I assumed the cat died; please confirm." The latter version would please my former editor who hated the word "that" as much as he hated the passive voice.
There are times when the passive voice is preferred (like in this sentence). C. Edward Good suggests flipping to the passive voice when one is trying to speak in the third person and has used the noun "one" too many times. Example: "Here are the eight situations where one prefers the passive voice" versus "here are eight situations where the passive voice is preferred. Another good example is when you want to hold the name of the actor until the end of the sentence as a "punchline." As in "the secret tapes were destroyed by the president" rather than "the president destroyed the secret tapes." For other exceptions, see Good's book at pp 126-129.