RewriteRule accepts numbers but not letters in tag

Leave .htaccess alone. You can use WordPress rewrite API for this.

add_action( 'init', function(){
    return add_rewrite_rule(
        'subdir/([^/]+)/?$', // ([^/]+) takes alphanumeric, while ([0-9]+) accepts digits
        'index.php?pagename=subdir&website=$matches[1]',
        'top'
    );
});

add_filter('query_vars', function($v){ return array_merge(array('website'),$v); });

First make sure subdir is a valid page slug, in this example. After you save that code to your child theme’s functions file or plugin, you can now use get_query_var('website') to get the website property rather than using $_GET global.

Hope that helps.