Do photographers provide model releases?
To upload a photo, the photographer agrees that they have the proper releases (model, trademark, etc.) needed to put the photo into the public domain. You can find more information on Unsplash’s Terms.
Please note that like any photo downloaded from a stock photography site (including Getty, Shutterstock, etc.), you are still responsible for making sure that you don't conflict with trademark or slander laws. You can read more about those laws here, which explains the role of the photographer, but more importantly, the role of the 3rd party in using any photo.