I am a little confused why
The Nature of Middle-earth is being used to critique the design choices of the
Rings of Power show. I have to imagine that most decisions related to the pre-production of the show (including the look of the Elves) were determined well before the publication of NoMe. So, the decisions were made when access to that information was simply unavailable. Furthermore, Tolkien fans' opinions on elven hair were also developed long before the publication of that book, so it's not like this new material is somehow groundbreaking or something we didn't already think about Tolkien's elves. To be fair, a lot of fan art and Peter Jackson's films also shape people's opinions on what elves ought to look like, and not all of that was based on what Tolkien wrote, either.
But I'm also curious why someone is starting with the most obscure references to establish whether or not Tolkien considered his elves to have long hair. It's not like their aren't specific examples in the texts.
Frodo and companions see Glorfindel for the first time on the way to Rivendell in
The Fellowship of the Ring:
"The rider's cloak streamed behind him, and his hood was thrown back; his golden hair flowed shimmering in the wind of his speed."
Here's the Fellowship meeting Celeborn and Galadriel in
The Fellowship of the Ring:
"On two chairs beneath the bole of the tree and canopied by a living bough there sat, side by side, Celeborn and Galadriel. They stood up to greet their guests, after the manner of Elves, even those who were accounted might kings. Very tall they were, and the Lady no less tall than the Lord; and they were grave and beautiful. They were clad wholly in white; and the hair of the Lady was of deep gold, and the hair of the Lord Celeborn was of silver long and bright; but no sign of age was upon them, unless it were in the depths of their eyes; for these were keen as lances in the starlight, and yet profound, the wells of deep memory."
Olwë is likewise described as having long hair, though to be fair this description appears in
HoME X: Morgoth's Ring (not the published Silmarillion):
"Two lords they had, for their numbers were very great: Elwë Singollo, which signifies Greymantle, and Olwë his brother. The hair of Olwë was long and white, and his eyes were blue; but the hair of Elwë was grey as silver, and his eyes were as stars; he was the tallest of all the Elven-folk."
The description of Fingon's long dark hair plaited with gold from "The Shibboleth of Fëanor" in
HoME XII: The Peoples of Middle-Earth has been known since 1996, and is incorporated into most fan art depicting Fingon after that date. Here's some samples:
"Cousins" by Ivanneth
"Rescue of Maedhros from Thangorodrim" by Kasiopeia / Catherine Karina Chmiel / Katarzyna Chmiel-Gugulska
"Wardens of the North" / "Siege of Angband" by Jenny Dolfen:
Coincidentally, all three images also contain Maedhros...who is also depicted with long hair by Ivanneth and Kasiopeia, who have a rather universal 'elves have long hair' standard in their artwork. Jenny Dolfen often depicts Maedhros in Beleriand and Fëanor with short hair, but other male elves with long hair. So, some variety for her depiction of the Noldor.
I recognize that "long hair" is subjective. How long is long? I think that Tolkien provides a clue concerning that in
Lord of the Rings. When Boromir arrives in Rivendell for the Council of Elrond, his hair length is described as 'his locks were shorn about his shoulders.' To me, this suggests that allowing hair to grow longer than shoulder-length is normal in Rivendell, so that Boromir's is particularly short compared to the others (despite his 100 day journey). Other interpretations are possible, naturally, but the use of 'shorn' here in particular makes me think that Boromir's hair is noteworthy for being short, rather than for being long. Either way, we know it's shoulder-length, which is more specific than we get in most descriptions!
Most artists tend to interpret 'long' as...very long indeed, as the examples above are quite typical rather than unusual when it comes to the representation of Elves in Tolkien fanart.