I think it would have been for the higher echelons. Definitely not for the servants and slaves.
Is there any lore to support this? It would make sense I guess considering everything, just wondering.
I'm not sure if they woke or not. Some sources imply they didn't, if it were truly uthenera. Other sources imply that they did. The elves in uthenera seem to have guided other elves through the spirit world, offering wisdom etc. to those who wandered the Fade (or whatever there was before the Veil). Perhaps uthenera is more of a permanent severing of spirit from body, whereas there were other forms of slumber that were less drastic? If uthenera severs body and spirit permanently, then basically it's immortality at the cost of the physical world and all that goes with it. It also means you might still be around even if your body were destroyed, though Felassan didn't seem to think that's the case.
Anyway... Abelas. I want his armor for Solas, next playthrough, for giggles. "Yes, be shiny hobo elf! You wear your clothes tight enough already... it shouldn't offend your modesty..." Too bad the Sentinel elves do not share their armor.
The Codex entry implies that waking up was possible, but rare. As for the body/spirit connection, the codex says "In time, the body would deteriorate and the elder would die in truth." The fact that the body deteriorating meant that the elf themself would die implies that the spirit was intimately connected to the body, no? Or hm. Maybe there's something I'm missing about the Fade and the afterlife.
Plus the whole "every time I wake up, the world seems more screwed up" thing. I think he's grumpy, too. I'm sure there's an angsty Lavellan out there, angry about Solas, that would be willing to help Abelas find a new name... and mood.
Well, I only say that I think it's uthenera because that's the only "stasis" sort of elven thing we've heard of so far. As for only the noblest elves partaking, well, they are Mythal's elite guard, so I can see that applying here.
As for immortality, like Tolkien's elves, it does not mean they cannot be killed. It simply means that they do not die of old age.
So if they're bound to the will of Mythas now, could that mean they are now potentially bound to the will of Solas (depending on what actually happened?)