While I agree with the general point that basic and direct sentence structure is not always the right choice, there is no passive voice in the sentence you quoted.
"it is a truth universally acknowledged" -> impersonal construction
"a single man... must be in want of a wife" -> this is active voice, man is still the subject of the sentence and be is the the only verb. "Want" is a noun. See https://www.dictionary.com/browse/want, definition 14.
"it is a truth universally acknowledged" -> impersonal construction
"a single man... must be in want of a wife" -> this is active voice, man is still the subject of the sentence and be is the the only verb. "Want" is a noun. See https://www.dictionary.com/browse/want, definition 14.