After observing some goings-on on my realm, and asking for other friends on other realms to verify, it seems that Elemental Invasions are missing.
In Vanilla, Elemental Invasions were added in Patch 1.4.0 (the same patch the Honor system was introduced, which is theoretically the patch we are currently on). Their primary feature was to spawn hordes of predetermined elementals in high-level zones: Azshara had Earth Elementals in the Ruined Reaches, Winterspring had Water Elementals in Lake Kel’theril, Un’goro Crater had Fire Elementals at Fire Plume Ridge, and Silithus had Air Elementals in the Crystal Vale. Elemental Invaders would pour forth from portals without end, until a raid-level boss mob (usually taking a very strong 5 man or, typically, 10-20 people) was destroyed, preventing any further spawning of Elemental Invaders. Elemental Invasions would also cause zone-wide emotes and yells, alerting players that they were in progress.
As World PvP is in full bloom and the prices of certain essences are high, I think Elemental Invasions would be great world events from time-to-time, like World Bosses but more casual. However, I have not had any confirmation that they have spawned on my server, nor do I have confirmation from contacts on other servers. Additionally, Wowhead does not have any information listed for the mobs or have their droprates recorded, meaning that nobody with the Wowhead client has seen them or looted them.
As World Bosses have spawned today, I do have some concern as to whether or not the Elemental Invasions are coming. None of the timelines mention when exactly it will be implemented, and not many players are aware of Vanilla’s Elemental Invasions in the first place, so it’s not unreasonable to think that it’s been forgotten about.
The spawn rate for every individual invasion is about once every three days, so it’s still possible that they are implemented; if you have any evidence of their occurrence on your server (such as a screenshot of the invasion in progress, one of the boss’ emotes, or a BoE item that only drops from the invasion boss appearing in Trade/AH), please share it in the thread.