An interesting question.
Frost chose more than one old-fashioned way to be new.
The simplest way is that he used traditional poetic structures to address modern concerns. This can be seen in " Stopping by Woods on a Snowy Evening."
Another old-fashioned way to be new is the accent or emphasis on nature, in a near pastoral. This can be seen in "Birches."
A third old-fashioned way to be new is the use of the unified narrator—the "I" in " The Road Not Taken."
A fourth is the relative lack of mention of new technologies in any of these.
These are all old-fashioned elements to poetry, but you could say they are new, or address the new, due to the themes of the poems.