Jack Antonoff co-produced Taylor Swift’s “august” and had previously worked with her on 1989.
Antonoff’s collaboration with Swift began before Folklore. He co-wrote or co-produced material on 1989, Reputation, and Lover, helping create a continuing musical partnership across different phases of Swift’s career. On “august,” their work produced a spacious arrangement built from guitars, keyboards, strings, and heavy reverb.
The track’s sound is often described through dream pop, guitar pop, and soft rock elements. Those classifications distinguish it from the bright synth-pop associated with much of 1989, even though Antonoff’s production relationship connects the two albums.
Aaron Dessner also co-produced much of Folklore, which makes him an understandable alternative answer. However, Dessner is not the collaborator specified by the earlier 1989 connection in this question. The correct producer is Jack Antonoff, who also performs and records with the band Bleachers.