There isn't any single source to corroborate this notion, it's primarily informed by two things:
1) The absolute majority of all High Elf NPC's are aligned with the Silver Covenant. I've obviously not counted, but you can count the number of High Elf NPC's that aren't a part of the Silver Covenant on your hands (maybe one foot, as well), whereas there are literally hundreds upon hundreds of Silver Covenant NPC's at this point.
2) It is stated in the Warcraft Chronicle that there had been schism, with both social and political origins, developing between the High Elves living in Quel'thalas and the High Elves living in Dalaran (and to a lesser extent, Lordaeron) for
centuries. It is this latter grouping which today comprises the vast majority of all modern High Elves, owing to the fact that they were far more concerned with helping their immediate allies (Lordaeron, and even Dalaran when it was temporarily sacked) instead of "returning" to a homeland that they'd been estranged from for so long that they likely didn't even remember it.
The point being that the recounting only deems it important to mention two specific groups, those High Elves living in/around Quel'thalas and those High Elves living in/around Dalaran. If there were other important groupings, with distinctive cultural proclivities, they likely would've been mentioned in any capacity.
Additionally, the Silver Covenant is implied to be composed of citizens from Dalaran. This is supported by the fact that the leadership of Dalaran has never once intoned that the Silver Covenant is in any way foreign to them. An important thing to note, because prior to the Warcraft Chronicle it was generally assumed by the playerbase that most/all of these High Elves living in/around Dalaran were slaughtered when the city fell -- which is why people came to conclude that Vereesa and all of her followers were one-time Blood Elves that had decided later that joining the Horde was a mistake. We now know, however, that Dalaran fell with almost zero casualties and virtually all of those resident High Elves not only survived, but fled to both Theramore and Stormwind before ultimately returning to Dalaran to assist in the rebuild.
The TL;DR being that if the Warcraft Chronicle deemed it worth mentioning that the High Elves were largely split into two camps, prior to the Sacking of Quel'thalas, and hints that the group domiciled in/around Dalaran were largely unaffected by the two calamitous events that are well-known to the playerbase (the fall of Dalaran, and the sacking of Quel'thalas). This alone presents modern High Elves in a light favorable to being made playable, but anybody trying to hand-wave away the concerns coming from Blood Elf players regarding an infringement upon their identity is a fool -- we're never going to get High Elves if they look even remotely like Blood Elves (and that is fine).