The Legacy category of achievements used to be for achievements that players had earned, but are no longer achievable in game if not already earned. When these achievements are not moved/removed from active category, they act like an itch in the brain to people who like to do completionist content.
I will provide 2 examples:
Azeroth at War: The Barrens - certain missions required for this achievement no longer spawn at the mission table. They were removed because they reward gold. Because this achievement is incompletable, it should be removed (or moved to legacy for those who completed it prior to shadowlands launch).
Wowhead reference:
Azeroth at War: After Lordaeron - certain missions required for this achievement no longer spawn at the mission table. They were removed because they reward gold. Because this achievement is incompletable, it should be removed (or moved to legacy for those who completed it prior to shadowlands launch).
Wowhead reference:
I suspect there are more examples of this, and if you’re reading this and have a similar brain itch to mine, feel free to reply with your example.
Let’s keep the achievements pane clean of achievements that can’t be completed!