Meta Tags are properties for Search Engine Optimization (SEO). So as long as you have the option of providing SEO properties for a Page, you will see the option to edit the tags. And along with that you will also find other properties, viz., Title and Description there.
If you hover over to the info icon on meta tag, you will see that information as below.
Optionally add <meta> tags to your page. These are used to provide information to search engines, browsers, and web services.
If you see the other pages, e.g., Contact Support, it allows you to set properties for SEO. Whereas in case of ArticleDetail, you will notice the text there which says, it has already been configured.
The search engine optimization (SEO) properties are automatically configured for this page
With what I see in the Community builder, the SEO option is not applicable/available for Pages of type:
- Article Detail