Blizzard's official September 23 explainer video says at 00:03:54: "The 40-player raid, Onyxia's Lair, is available at launch."
That wording conflicts with Blizzard's September 12 What's Next recap, which says new raids unlock on December 9. Both are first-party sources. Treat Onyxia as a launch-raid claim, but keep the exact availability date open until Blizzard clarifies which roadmap applies.
The video also says most raids use 10- or 20-player groups and that more 10- and 20-player experiences arrive with content updates. It gives no new Server Slam date or level-cap change.
SOURCES: Blizzard's official video and What's Next Panel Recap.