The getSummary() method has been added to Drupal\Core\Field\FieldTypeCategoryInterface, and it is implemented in Drupal\Core\Field\FieldTypeCategory.
If a class implements FieldTypeCategoryInterface and does not extend FieldTypeCategory, then it will have to be updated to implement the new method.
When the site builder adds a new field from /admin/structure/types/manage/page/fields (for example) and chooses a field-type category, the text from getSummary() will be shown on the form. For example, the options module adds the summary text below the Label field in this screenshot:
A module that adds a field-type category can provide summary text by adding the summary key to the YAML file. For example, the summary text in the screenshot comes from core/modules/options/options.field_type_categories.yml:
selection_list:
label: 'Selection list'
description: 'Create text or number based list with predefined options'
summary: 'Each list item has a Name, with limited formatting, and a Value. Values that are in use cannot be changed, since they are stored in the database. The Names can be edited later.</p><p>The field type examples use the format Name (Value): Name is displayed to the user, and Value is stored in the database.'
weight: -15
libraries:
- options/drupal.options-icon
It is safe to add the summary key to mymodule.field_type_categories.yml: if mymodule is used with earlier versions of Drupal core, then the summary will be ignored.
Like the values of the existing label and description keys, the new value is translatable.
