No, that’s not at all true. The plural of elf has always been elves in Modern English. You’re probably thinking of dwarves, but even that has some precedent. Old Norse had the plural įlfar in the Eddaic sagas. From Beowulf, in Old English dating from anywhere between the 8th and 11th centuries, we have ylfe meaning elves in
Originally Posted by Midgardsormr
Fram žanon untydras ealle onwocon eotenas and ylfe.
Note that that earlier word in that sentence, eotenas, meant giants in Old English, is the word Tolkien updated to use for his ents.
Perhaps as a merger of įlfar and ylfe, around 1000 a.D. in Saxon Leechdom we find the singular ęlfe in
Wiš ęlfe and wiž uncužum fidsan gniš myrran on win.
The plural alven is first attested in 1205. Around 1386, Chaucer used singular elf in The Man of Law’s Tale:
The mooder was an elf by aventure.
Gavin Douglas (writing in Scots) uses elvis for the plural in his Eneados of 1513, which was a Scots translation of Virgil’s Ęneid. Finally in 1553 we have Nicholas Udall using the modern form elves (‘mad peevish elvesf’) in his Royster Doyster, and many others shortly thereafter. The preacher Henry Smith in 1593 used elves this way:
Frenzies, furies (wayward elves): What need ye call for whip or scourge?and in 1610, Shakespeare in The Tempest used elves this way:
Ye Elves of hills, brooks, standing lakes and groves.
The tale of dwarfs versus dwarves is different, and well recorded elsewhere. Modern dictionaries now regularly cite both forms for the plural, a testament to Tolkien’s great influence in these things. Before Tolkien, nearly no one used dwarves that way, and now it is perhaps the more common usage for the Eddaic sort of mythological creature; dwarfs if used at all is used for people afflicted with dwarfism. When Tolkien used it, it was still considered an error. The earliest modern use regularly cited before Tolkien is from 1818, when William Taylor wrote:
The history of Laurin, king of the dwarves.
Still, before Tolkien it was considered an error; now it is everywhere. In Tolkien’s letter to Stanley Unwin (#17 of Letters, pp 23-24), he wrote:
The real historical plural of ‘dwarf’ (like teeth of tooth) is dwarrows anyway: rather a nice word, but a bit too archaic. Still I rather wish I had used the word dwarrow.
The reason he says that is that in Old English, dwarf was dweorh, and of similar form to this, beorh meaning ‘hill’ came down to us as barrow, as in the Neolithic long barrows. Tolkien used barrow in his Barrow‐downs and Barrow‐wights. And he did come to use dwarrow in the ancient sense, after all. The Dwarrowdelf (ie, a ‘Dwarf-delving’) was the common name for Khazad-dūm, those great mansions of Durin’s Folk which the elves called Moria, the black pit.
Far to the east were the most ancient dwellings of the Naugrim…Greatest of all the mansions of the Dwarves was Khazad-dūm, the Dwarrowdelf, Hadhodrond in the Elvish tongue, that was afterwards in the days of its darkness called Moria; but it was far off in the Mountains of Mist beyond the wide leagues of Eriador, and to the Eldar came but as a name and a rumour from the words of the Dwarves of the Blue Mountains.
― The Sillmarillion, ‘Of the Sindar’