When I wrote the essay, Misguided, I assumed the Romeo and Juliet story was fictional. But when I looked into the matter, I found a lot of confusion. Wikipedia doesn’t answer the question but its information does help explain why the guide books were confused. Of the several plays that predate Shakespeare’s (which was written between 1591-95), the earliest two claim their tales were based on a true story. The first was by Masuccio Salernitano, who included much of the final plot—“the secret marriage, the colluding friar, the fray where a prominent citizen is killed, [the hero’s] exile, [the heroine’s] forced marriage, the potion plot, and the crucial message that goes astray”—and who claimed to have based his story on an incident that occurred in his lifetime. It is possible that “based on a true story” had the same commercial value that it does today, so Salernitano’s avowal may have been merely savvy. But it might conceivably have been true.
The next iteration, by Luigi Da Porto, used the names Romeo and Juliet, the Montagues and Capulets, and the setting of Verona. Da Porto insisted that the historic basis of his play took place a hundred years before Salernitano. How would he know that? Clearly, the further from an event, the less credible an account, so Da Porto’s claim seems particularly dubious. The two stories have too much in common for both to have been independent events; and if the older story is the original “true” version, why were the names Romeo and Juliet and the setting of Verona not kept by Salernitano?
Later versions of the play dropped the claims of an historic precedent. Matteo Bandello”s play of 1554 introduced the nurse and Benvolio. It was translated into French in 1559 by Pierre Boaistuau and, in turn, into English by Arthur Brooke, who wrote in verse. Brooke’s poem is the basis of the play by Shakespeare, who presumably took the story to be fiction.
In a general sense, the story is surely “true.” For centuries, young people have fallen in love across a cultural/societal Maginot Line and paid a terrible price. They continue to do so today.