Seeing some of the Lava Divers achievements made me want to have a list all in one place of what's been done for Legendary raids. So I looked through this forum section, and compiled what I found. Please add the information if you know of an earlier R10 completion, or a higher-difficulty completion, than what I have listed.
Legendary raids, either earliest known R10, or highest-difficulty known completion, as of 2021-03-05:
The Dryad and the Demigod
R10, 2021-03-05, Lava Divers (Khyber)
Killing Time
R10, 2021-01-16, Lava Divers (Khyber)
Legendary Vision of Destruction
R10, 2021-01-15, Ascendance (Cannith)
Too Hot to Handle
R10, 2020-12-21, Lava Divers (Khyber)
Legendary Lord of Blades
R10, 2020-12-03, Ascendance (Cannith)
Legendary Master Artificer
R10, 2020-11-17, Lava Divers (Khyber)
Project Nemesis
R10, 2020-08-10, Lava Divers (Khyber)
Defiler of the Just
R10, 2020-07-15, Lava Divers (Khyber)
Riding the Storm Out
R10, 2020-06-14, Lava Divers (Khyber)
Legendary Hound of Xoriat (not counting any completions prior to 2019-08-13 Reaper re-opening in U42.4, as mechanics were changed drastically)
R10, 2020-07-19, Lava Divers (Khyber) (and also on 2020-04-24, no link provided)
Fire on Thunder Peak
R10, 2020-03-01, Lava Divers (Khyber)
The Mark of Death
R10, 2020-03-01, Lava Divers (Khyber)
Temple of the Deathwyrm
R10, 2020-02-23, Lava Divers (Khyber)
The Codex & the Shroud
R10, 2020-01-19, Lava Divers (Khyber)
Old Baba's Hut
R10, 2018-10-07, Omnipresence (Ghallanda)
The Curse of Strahd
R10, 2018-07-14, Helter Skelter (Ghallanda)
Legendary Tempest's Spine
R10, 2017-02-04, Band of Gypsys (Khyber)