Sorry, I don’t use MySQL so I thought 8.4 was the last version of 8.x but seems there are two branches.
The version is not the right one, checking the repo I see you should use mysql-8.4-lts instead of mysql-8.4
echo "[ * ] Mysql 8.4"
echo "deb [arch=$ARCH signed-by=/usr/share/keyrings/mysql-keyring.gpg] http://repo.mysql.com/apt/debian/ $codename mysql-8.4-lts" >> /etc/apt/sources.list.d/mysql.list
echo "deb-src [arch=$ARCH signed-by=/usr/share/keyrings/mysql-keyring.gpg] http://repo.mysql.com/apt/debian/ $codename mysql-8.4-lts" >> /etc/apt/sources.list.d/mysql.list
Remember to include the other echo "deb lines, the ones that don’t specify the version.
No, there is no need to add anything else.