Is it possible to modify help text on a vocabulary page? I am using a custom way of managing relations between terms in a particular vocabulary so I want to disable (or at least hide) the built-in functionality allowing terms to be placed in a hierarchy.

I have managed this to some extent with CSS (managed by the module that also does the custom relations). I have removed the "Relations" section from the taxonomy term edit form and I have removed the handles from the vocabulary list so that you cannot move the terms around and place them in a hierarchy that way. But I cannot see how to get rid of, or alter, the help text which says "You can reorganize the terms in Topic using their drag-and-drop handles, and group terms under a parent term by sliding them under and to the right of the parent.".

The problem is that there are no containers around this to identify which vocabulary is being adminstered, or indeed that it is on a taxonomy vocabulary list page at all. I have read the Drupal 8 docs on attaching libraries and they all say that targeting pages by URL is deprecated.

I suspect hook_form_alter would not work either as it is outside of the form tags.

Is there any way to do this?
When creating a vocabulary you have the option of turning on/off the ability to arrange terms in a hierarchy. If you choose "Single Hierarchy" for example then the help text will change to read: "VOCAB contains terms grouped under parent terms. You can reorganize the terms in VOCAB using their drag-and-drop handles."

See http://cgit.drupalcode.org/drupal/tree/core/modules/taxonomy/taxonomy.module#n78

... Otherwise you can create a custom module that implements hook_help and check for $route_name = entity.taxonomy_vocabulary.overview_form. As long as your module's hook_help fires after taxonomy_help() your's should win. See https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Extension%21module.api.php/function/hook_module_implements_alter/8.4.x to control whose module wins when implementing a hook