How to Disable WP JSON Rest API in WordPress without plugin ?

just add below code in wordpress theme function.php file at the bottom

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Add the following code to functions.php (anyone)
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if( ! is_user_logged_in() ) {
remove_action('rest_api_init', 'create_initial_rest_routes', 99);
}


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OR
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remove_action('rest_api_init', 'create_initial_rest_routes', 99);

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Check if your site's username is exposed
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website-URL.com/wp-json/wp/v1/users
website-URL.com/wp-json/wp/v2/users
website-URL.com/wp-json/

remove_action('rest_api_init','create_initial_rest_routes',99);