ここで書いている情報に従ったらうまくいった．
sudo apt purge brave-browser brave-keyring sudo rm /etc/apt/sources.list.d/brave-browser-release.list sudo apt update
現象
apt update を実行すると，以下のエラーが発生(厳密にはワーニング)．brave browserを数日前にアンインストールしたなぁと思って色々ぐぐってみた．
GPG error: https://brave-browser-apt-release.s3.brave.com stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A8580BDC82D3DC6C W: Failed to fetch https://brave-browser-apt-release.s3.brave.com/dists/stable/InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A8580BDC82D3DC6C W: Some index files failed to download. They have been ignored, or old ones used instead.
