Onyxia is dead.

Everyone is. It's the same kind of stuff that drops off instances that are 10x easier.

Considering Legendary and Artifact item qualities aren't in yet, I would say they may be placeholders...

So this hasn't been confirmed yet, I take it?

Blizzard has let on that she had indeed been killed, they were looking into the legitimacy of the kill.

No updates past that, unless I missed it.

How long would it take for a monster of that stature to respawn?

Five days.

I thought she could only be ATTEMPTED every 5 days by the people in the group.

If it's a 5 day respawn, that's going to SUCK a long time down the road. People will be camping the portal, waiting for the respawn so they can get her.