All in One SEO no longer generates its own robots.txt as a dynamic page. Instead, it uses the robots_txt filter in WordPress to tap into the default robots.txt created by WordPress.
When you use the Robots.txt feature in AIOSEO, you're modifying the default robots.txt created by WordPress.
Rewrite rules should no longer be needed for NGINX servers. However, if you're seeing a 404 error when browsing to your robots.txt, then the NGINX rewrite rules may be incorrect. Check with your hosting provider that the rule below is present in the NGINX config file for your server:
rewrite ^/robots.txt$ /index.php?aioseo_robots_path=root;