Harry and Taylor wrote songs together:
Here's some "proof", if you will.
https://www.hotpress.com/Taylor-Swift/features/reports/Cover-Story-The-Making-of-Taylor-Swift/22348300.html
Since being released, the website has made it where you can only read the article if you subscribe to the website but I have some screenshots.
this was a quote from a mutual friend, which states that harry and taylor did indeed write songs together maybe around 2014.
here's some fan theories,