From sleuthing of Phoebe’s Instagram, as well as scanning the internet, it appears that she is single.
The last person she was linked to was Alex Greenwald, the lead singer of the band Phantom Planet and Brie Larson’s former fiancé. The above picture was posted on June 29, 2020, making Phoebe and Alex Instagram official. However, the picture has since been deleted and there isn’t any other evidence of him, although we can’t find an official break up statement either.
Source: https://www.distractify.com/p/phoebe-tonkin-dating