Here goes example, "The earliest recorded use of "queer" as a form of homophobic abuse is said to be a 1984 letter by John Sholto Douglas, the Marquess of Queensberry."
[be] "said to be" is a passive locution used to assert something as a fact supported by authority without actually specifying that authority.
Notice that your example is well chosen.
It asserts as historical fact that no one used "queer" as a written slur against homosexuals before John Sholto Douglas, notoriously responsible for the prosecution and incarceration of Oscar Wilde. It is eminently plausible that Douglas used the slur in writing. It is not inherently plausible that Douglas coined the term. We cannot judge the credibility of whoever asserted these propositions because no name is provided; in fact we cannot even determine whether the person who allegedly said them is historical or fictional. We cannot judge the weight of the evidence because we are provided neither evidence nor a citation to what is implied to be evidence. Ultimately, even the author refuses to affirm these propositions and attributes them to some mysterious other.
"Said to be" is a rhetorical device implying the truth of a proposition without personally affirming it and without providing evidence or authority for it. All writers should have it in their rhetorical armamentarium, and all readers should view it with suspicion.