It’s virtually impossible to pinpoint when the misuse of a word or phrase becomes so common that it’s no longer deemed a misuse. But if mere repetition were the main criterion, “alleged” would have lost its stigma a long time ago.
When someone is arrested or accused of a crime, journalists commonly refer to that person as the “alleged murderer,” or the “alleged thief.” That use is decried by many, some of whom say that using “alleged” to modify the word “murderer” or “thief” is no more defense against libel than using the adjective “cute” would be.
In these cases, “alleged” is synonymous with “suspected.” But “suspected” means “viewed with suspicion,” while “alleged” means “so declared, but without proof or legal conviction.” In other words, calling someone “an alleged thief” is all but saying “we know you did it.”
So there is some basis for avoiding the use of “alleged” as the noun modifying the person. (If it’s unclear that a crime occurred, it could be called an “alleged theft,” modifying the crime.)
Interestingly, few usage guides address the issue directly. Merriam-Webster’s Dictionary of English Usage says the usage of “alleged murderer” is an Americanism, but is unclear whether it should be avoided, though it seems to lean that way. The Associated Press stylebook implies that “alleged” should not be used with the person, but never says it explicitly. Part of its entry on “allege” reads: “Do not use alleged to describe an event that is known to have occurred, when the dispute is over who participated in it. Do not say: He attended the alleged meeting when what you mean is: He allegedly attended the meeting.” Since in practically every crime the dispute is over who participated in it, one could infer that the AP frowns on the use of “alleged” in front of “murderer” or “thief.”
The New York Times stylebook is a little clearer: “Alleged and allegedly are police-blotter jargon, best rephrased into conversational English: accused of, charged with or suspected of. If legal concerns leave no choice, apply the modifier to the offense, not the suspect: alleged theft, not alleged thief.”
The use of adverbs or verbs applied to the crime, and not to the person, will help writers avoid any legal entanglements. “She is accused of robbing the bank,” “Police allege he broke into the church,” and “She is suspected of taking a bribe” all work fine; while there’s nothing legally wrong with “He is alleged to have murdered his mother-in-law,” it’s clunky.
As for the person, “the suspect” or “the accused” usually works quite nicely. (It’s redundant to call someone an “alleged suspect.”)
So now that you know not to use “alleged” to describe a suspect, and now that you have alternatives, we will never see “alleged murderer” again, right? Would that it worked that way. The use of “alleged murderer” is so deeply ingrained in journalistic culture, and the objections to it so imprecisely stated, that there are probably no courts of appeal left.