It is possible that Frodo's arrival in Valinor is an accurate recording of factual events in-universe
While, as far as we understand, there are no more immigrants from Valinor (only those heading to Valinor) who could bear tale of Frodo's arrival, there is at least one way for this to be an accurate factual account in Middle Earth.
Aragorn, who at the start of the Fourth Age possesses a palantír, both loved Frodo (friend, companion, hero), Bilbo (old friend), Gandalf (old friend, advisor, leader), Elrond (father in-law, father-like figure), and Galadriel (grandmother in-law, revered Noldo), and had motivation, values, capacity, and right to see visions of the undying lands, may have watched from afar as the ship from the Havens passed into the West. (In Unfinished Tales a note describes one of the stones as being specifically 'oriented' so that it always gazed to "Eressëa in the vanished West," implying that other stones might be directed that way also.) So the end of Frodo's journey may thus actually have been recorded as an in-universe fact which was subsequently passed along to the remaining hobbit members of the Fellowship, for example, when King Elessar visited the Shire, as recorded in the Appendices in Return of the King.