Refine snippet variable suggestions list on Search Appearance

[IreneStr]

In #10162, we've filtered out a lot of unwanted snippet variable suggestions for specific snippet editors on Search Appearance. However, we might be able to refine the lists further.

Open questions

• Currently, %%tag%% shows up for all taxonomies (categories, custom non-hierarchical taxonomies). Is this intended?
• Custom taxonomy names and descriptions show up for all custom taxonomies snippet editors. For example, if you have a movie custom taxonomy and a book custom taxonomy, "Movie (custom taxonomy)" and its description also shows up in the books custom taxonomy snippet editor.
• Should custom taxonomy names and descriptions also show up for custom post types?
• Should term404 also show up for pages?

[afercia]

One more (a subtle one) is: should "Archive title" show up for post types (and related taxonomies) that don't have a real archive? I.e. with has_archive set to false.

For example, what is the archive for posts and pages? It's true that WordPress fallbacks to an archive title "Archives" but they have has_archive set to the default false. Instead, for some custom post types is set to true e.g. Jetpack:
http://build.wordpress-develop.test/portfolio/
http://build.wordpress-develop.test/testimonial/

[jdevalk]

Currently, %%tag%% shows up for all taxonomies (categories, custom non-hierarchical taxonomies). Is this intended?

No.

Custom taxonomy names and descriptions show up for all custom taxonomies snippet editors. For example, if you have a movie custom taxonomy and a book custom taxonomy, "Movie (custom taxonomy)" and its description also shows up in the books custom taxonomy snippet editor.
Should custom taxonomy names and descriptions also show up for custom post types?

Hmm, is that hard to do? If so: no, otherwise: yes.

Should term404 also show up for pages?

No, it should only be available for 404 pages, because... Well, if it's a 404, a 404 page should be served, right?

One more (a subtle one) is: should "Archive title" show up for post types (and related taxonomies) that don't have a real archive? I.e. with has_archive set to false.

No. We special case two cases there that I know of: posts, with the blog page, and products, with the shop page, for woocommerce.