Legacy loot is being enabled by the weekly maintenance on March 31, according to a recent forum post by Community Manager, Kaivax. The post cites that they’ve reached a point in the expansion which does not require a full group any longer,
“With this change, Legion raid and dungeon encounters will drop a fixed quantity of class-agnostic loot. Today, if you and one other player enter Emerald Nightmare on Mythic difficulty and kill Nythendra, it’s unlikely that you’ll receive any loot, since drop rates normally scale to the number of players. After this change, Nythendra will always drop 5 items from her loot table, regardless of your group size.”
It continues, explaining that dungeons and several lower difficulty Legion raid bosses are already soloable if you’re a well-equipped Battle for Azeroth character, however, some more difficult bosses may still require a group of you and your friends.