From Allara's expansion checklist:

• Reevaluate group sizes for all items (items from 2 expansions ago become soloable; items from last expansion become groups of 3) - Note: Only content <= 10 levels below the player uses Personal Loot in BFA

Not sure if the BFA dungeons should be considered solo-able yet, but they'll be soon enough. I'll leave in the world boss and Island Expeditions, though. Everything else should be done as of a1fe131.