Generally speaking "Defamation is any wrongful injury to the reputation of a person or entity."
Concerning law , defamation (calúnia, defamação) can be separately dealt (tought, I mean), libel being written or otherwise published. Whereas the slander is the spoken.
Grammatically if the rumour ended, or the guy died (so, the the false rumours aren´t more into play) you say WAS, not HAVE BEEN as in:
"It was widely rumoured that he was involved with corrupt police and organised crime, collecting vast amounts of money through bribes."
Here the effect of defaming is into play at the time of the news, this effect is not over:
She advised Lukuku that in order not to waste the court's time, he should search for newspapers in which Mumba had allegedly been defamed.
(translation: ele deve procurar os papéis nos quais Mumba foi supostamente defamado/tem sido defamado. vês? não é a tradução em português em si, mas se o efeito persiste na época em que essa frase foi publicada)
Have been/has been may be used to give a certain untemporal and neutral effect in official documents =>
General or compensatory damages, which a court may award for a person's loss of reputation, shame or hurt feelings. Under common law, once the court has found that he has been defamed, the plaintiff does not have to prove that actual harm has been done.
Again, in Portuguese the translation would be either "uma vez que a corte descobriu que ele foi defamado" or "tem sido defamado" What matter is that the "effect isn´t over".
Of course, there are other ways to say one is/have been defamed, as in following item =>
Could Tom Cruise Sue "South Park" For Suggesting He is Gay? And Even If He Could, Should He?
"The same is arguably true of a claim by a straight person that he has been falsely labeled as gay: Such a claim takes advantage of the courts so that one person can escape bias that others unfairly suffer."
Or that he has been smeared etc etc. Has been gives an idea of continuity/business non finished [in English].