mysql_native_password is gone, and the user that used it

An 8.4 container image, an application that could not connect, and a plugin that was disabled by default rather than removed.

$ mysql -e "SELECT user, host, plugin FROM mysql.user"
  app      10.0.%    mysql_native_password
  repl     10.0.%    caching_sha2_password

$ # the error, which is clearer than most
  ERROR 1524 (HY000): Plugin 'mysql_native_password'
  is not loaded

$ mysql -e "ALTER USER 'app'@'10.0.%'
            IDENTIFIED WITH caching_sha2_password BY '...'"

The plugin can still be loaded with a startup flag, which is the escape hatch and is a way of deferring the change to a version where it is genuinely gone. Changing the authentication method also requires the client to support it — an old PHP build without the right handshake fails differently and less clearly, which is worth checking before the upgrade rather than during.