Achievements > Feats of Strength, lists the following achievements that is now unobtainable:

• Dragonflight Season 2 Hero -- Removed with the release of DF S3
• You're Really Doing It Wrong (Level 90) -- Removed with the release of Legion

Achievements > Legacy > Raids, lists the following achievements that is now unobtainable:

• Deadliest Catch -- Removed with the release of Cataclysm, the old ZG recipes re-released in 10.0.7 has a new achievement called "Deadliest Cache"

Seems like this should be resolved if the data is correct.

Dragonflight Season 2 Hero

• Still obtainable via the Mythic Raid
  You're Really Doing It Wrong
• achievement=9597 Has comments stating its still obtainable the level restricted achievement "8812" is marked as never implemented
  Deadliest Catch
• Was timelined out in version 3.8.4