Most recently unified, certainly - I thought that The Lost Years was incorporated indirectly by the current continuity, though. (Gateways -> Rough Trails -> Traitor Winds -> The Lost Years)
Well, just because books reference elements of each other, that doesn't necessarily make them in continuity. The middle two Lost Years
books, A Flag Full of Stars
and Traitor Winds
, directly contradict each other in their chronology because they were from different editors and came out during a time when arranging any continuity at all between novels was an uphill battle. And they don't necessarily fit too well with TLY itself for the same reason.
And Traitor Winds
isn't really compatible with TOS canon, because it ignores the existence of the Romulan-Klingon technology exchange established in "The Enterprise Incident." I've always thought of it as an alternate-universe tale because of that.
I think it's best just to let the '80s continuity be its own thing rather than trying to shoehorn it into the modern one. The underlying assumptions are too different because so much new canon has been laid down in the interim. And The Lost Years
is pretty firmly integrated into the '80s continuity, since it continues character threads from Dillard's previous three novels and directly references the events of Dreadnought!
I think it's better to let that stand as its own distinct interpretation of the Trek universe. After all, it's all equally imaginary, so it's not like one version is more "right" than another. Being incompatible with canon, or with the modern novels, doesn't make the '80s books less worthwhile or less valid. It just makes them a different creative direction, one with its own distinct character.