As the error states, WordPress has deprecated the login_headertitle function. You most likely have a plugin or parent theme that is using this function to change the title on your login page. Check to see if there are any updates to your plugin and theme files.
Seeing this message also indicates that you have debugging enabled on your site. If it’s a live site that you are not developing on, you probably want to disable that. Or, you can keep debugging enabled for development and have it log to a file instead of displaying on the screen. WordPress.org has documentation on debugging.
If you’re developing a theme or plugin that is using login_headertitle, it’s easy to resolve. If there’s an add_action or another function call to login_headertitle, just replace it with login_headertext. Here’s an example from the WordPress Code Reference for the login_headertext function:
// Customize login header text.
add_filter( 'login_headertext', 'customize_login_headertext' );
function customize_login_headertext( $headertext ) {
$headertext = esc_html__( 'Add custom login page text here', 'plugin-textdomain' );
return $headertext;
}