"Niall diss track incoming," one fan wrote on Twitter, while another suggested the track was a "breakup song??"
Prior to the big announcement, Hailee shared photos from 2019 on her Instagram Stories with the note: "Thankful for all the lessons, the love, the heartbreak, & the memories."
She and Horan reportedly called it quits on their romance months ago with sources telling Us Weekly the couple struggled to find time for one another, but remain friends.
They were first linked in January 2018, but neither spoke about the dating rumours, with Hailee telling Us, "I feel so lucky that they (fans) care enough about me to know what I'm doing every second of every day, but I think that sometimes it's hard for people in general to realise that there are boundaries to a person's life and personal life. I am a very private person."
Hailee and Niall were photographed kissing for the first time in August 2018.
Horan confessed he had just been through a painful break-up earlier this month, revealing his new song "Put a Little Love on Me" was inspired by the heartbreak.
"I'd just gone through a breakup and it was all very real. It was very easy to sit down at the piano and speak and see what happens," he told The Sun.