For Debian Sid run the following:
Keep in mind that the owner of the key may distribute updates, packages and repositories that your system will trust (more information).
echo 'deb http://download.opensuse.org/repositories/home:/ungoogled_chromium:/testing/Debian_Sid/ /' | sudo tee /etc/apt/sources.list.d/home:ungoogled_chromium:testing.list
curl -fsSL https://download.opensuse.org/repositories/home:ungoogled_chromium:testing/Debian_Sid/Release.key | gpg --dearmor | sudo tee /etc/apt/trusted.gpg.d/home_ungoogled_chromium_testing.gpg > /dev/null
sudo apt update
sudo apt install ungoogled-chromium