The upstream repository has moved. The latest one can be found here: https://brave-browser.readthedocs.io/en/latest/installing-brave.html#debian-9-ubuntu-14-04-and-mint-17
The repo itself is: deb [arch=amd64] https://brave-browser-apt-release.s3.brave.com/ stable main and the signing key is available from: https://brave-browser-apt-release.s3.brave.com/brave-core.asc Francois
