So... is D-1 Devastator finished or not?
NO, it is not finished.
Why?
MangaUpdates contains information about the anime adaptation and indicates that it is incomplete.
Watching Order (by release date)
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Synopsis
Ryo, driven by the disappearance of his friend Masato, joins the Yaesu vehicle company. He seeks answers about Masato's fate at the Makuhari test course, where Masato was last known to be.
He is offered a chance to learn about Masato by testing a new vehicle, the Vector Versus, late at night.
While testing the machine, Ryo experiences an unexpected and extraordinary event: he's transported to an unknown dimension.