Harry Styles is an ex-boyfriend of Taylors from the Red and 1989 era. They dated for a few short months but are very popular amongst fans because of the notion that both are huge singers and artists and some of the most famous. Some fans even refer to them as "Mom and Dad" and that they are "Children of Divorce". Despite being exs, media coverage from specifically award shows have shown the support both artists give to each other.
In the Red Secret Sessions, Taylor makes a note that Treacherous is about the same person who I Knew You Were Trouble is about
Taylor won a VMA for this song in 2013 and personally called out the person who it was about like they were in the audience…Harry styles was attending with One Direction
This song has a lyric parallel to Out Of The Woods in 1989, which is very believed to be about Harry
It’s a little obvious…he also matched very well to the description of the male in the chorus
The song refers to a car crash due to the male crashing the car…and it’s pretty well known that Harry is not a good driver
Pictures were released of Harry and Taylor dancing on some kind of stage in New York, and barefoot, which both those facts are mentioned in the song. Taylor also dated Harry through her Red/1989 Era (and she moved to New York for 1989, per the beginning song of the album, Welcome To New York)