Remove Yoast SEO version from header

functions.php

add_action('get_header', 'start_ob');
add_action('wp_head', 'end_ob', 999);

function start_ob() {
	ob_start('remove_yoast');
}
function end_ob() {
	ob_end_flush();
}

function remove_yoast($output) {

	if (defined('WPSEO_VERSION')) {

		$targets = array(
			'<!-- This site is optimized with the Yoast WordPress SEO plugin v'.WPSEO_VERSION.' - https://yoast.com/wordpress/plugins/seo/ -->',
			'<!-- / Yoast WordPress SEO plugin. -->',
			'<!-- This site uses the Google Analytics by Yoast plugin v'.GAWP_VERSION.' - https://yoast.com/wordpress/plugins/google-analytics/ -->',
			'<!-- / Google Analytics by Yoast -->'
		);
		
		$output = str_ireplace($targets, '', $output);
		$output = trim($output); 
		$output = preg_replace('/^[ \t]*[\r\n]+/m', '', $output);
	}
	return $output;
}

Hi all... This is how I got rid of the two html comments. Code in my functions.php: (latest WP and Yoast SEO)

if (defined('WPSEO_VERSION')){

    $instance = WPSEO_Frontend::get_instance();

    remove_action( 'wpseo_head', array( $instance, 'debug_marker' ), 2 );

    remove_action( 'wp_head', array( $instance, 'head' ), 1 );
    add_action( 'wp_head', 'custom_yoast_head', 1 );

    function custom_yoast_head() {

        global $wp_query;

        $old_wp_query = null;

        if ( ! $wp_query->is_main_query() ) {
            $old_wp_query = $wp_query;
            wp_reset_query();
        }

        do_action( 'wpseo_head' );

        if ( ! empty( $old_wp_query ) ) {
            $GLOBALS['wp_query'] = $old_wp_query;
            unset( $old_wp_query );
        }

        return;

    }

}

Original head() function in class-frontend.php of the plugin is this:

public function head() {
    global $wp_query;

    $old_wp_query = null;

    if ( ! $wp_query->is_main_query() ) {
        $old_wp_query = $wp_query;
        wp_reset_query();
    }

    /**
     * Action: 'wpseo_head' - Allow other plugins to output inside the Yoast SEO section of the head section.
     */
    do_action( 'wpseo_head' );

    echo '<!-- / ', $this->head_product_name(), ". -->\n\n";

    if ( ! empty( $old_wp_query ) ) {
        $GLOBALS['wp_query'] = $old_wp_query;
        unset( $old_wp_query );
    }

    return;
}

This is very helpful! Thank you. It works perfectly in a theme's function.php - however, is there any way to add this to a plugin? I've tried without luck. I appreciate any insight.

Fusse's example works Great. Thanks Fusse!

But keep in mind NO MATTER which method you use it will BREAK forced title rewrites! (At least in the version I am using, 3.4.2)
They use the "debug_marker" (The comments) to place a the forced title rewrite. If you are not using "force title rewrite" then you should be fine.... For now.
// Find all titles, strip them out and add the new one in within the debug marker, so it's easily identified whether a site uses force rewrite. $content = preg_replace( '/<title.*?\/title>/i', '', $content ); $content = str_replace( $this->debug_marker( false ), $this->debug_marker( false ) . "\n" . '<title>' . $title . '</title>', $content );

As you can see they use the opening comment tag as a point to insert the title tag.

Or...just use another plugin :)

We’ve Migrated from Yoast SEO - https://roots.io/weve-migrated-from-yoast-seo/

Hello,
How to remove this line from inspect element.