MySQL  8.0.17
Source Code Documentation
Clear text client plugin
  • This client side plugin is used by a number of server plugins: LDAP (authentication_ldap_simple) and PAM (authentication_pam) to name a few.
  • The client name is mysql_clear_password
  • Client side requires nothing from the server. But the server generates and sends a 20-byte Native Authentication compatible scramble.
  • Client side sends the password in clear text to the server
Sending the scramble is not necessary for the clear text method, but, since the server always initiates the exchange by sending Protocol::Handshake and that one has a placeholder for authentication plugin dependent data the server does fill that space with a scramble should it come to pass that it will back down to Native Authentication. This is also why it's OK no to specifically read this in clear_password_auth_client since it's already read as a part of the initial exchange.
See also
clear_password_auth_client, server_mpvio_write_packet, send_server_handshake_packet