Long before I discovered the very useful Query Monitor plugin I wrote a simple version that put information about a WordPress page/post in a comment.
The information was very useful during theme development. Just like Query Monitor, it would tell me the template tags, e.g. is_home(), is_singular(), is_archive() etc., that are active for that page and the template file that is being used.
Unlike Query Monitor it displays the status of WooCommerce template tags, e.g. is_cart(), is_product() etc., and featured image sizes. I have considered converting them to work within Query Monitor but the code required looks intimidating.
The code is only run for the specified IP address, or logged in administrators.