Why don’t my custom taxonomies show up in WordPress’s post edit pages?

If you use custom taxonomies in WordPress, they don’t automatically get added to the new Gutenberg block interface.

Fortunately this is an easy fix: you need to add this extra parameter to the arguments you set when you first define your taxonomy:

'show_in_rest' => true

so your taxonomy arguments might look like this:

$args = array(
    'hierarchical' => false,
    'public' => true,
    'show_ui' => true,
    'show_admin_column' => true,
    'show_in_nav_menus' => true,
    'show_in_rest' => true,
    'show_tagcloud' => true,
    'show_admin_column' => true,
);

And your post edit screen should now show your taxonomy: