I want to reset all user roles back to those that come default in WordPress.
When I de-activate your plugin the roles seem to stay.
I want to re-set them so I can start again.
Users’ capabilities are stored in the database in the wp_user_roles option in the wp_options table, so you need to update this option directly in the database. You can use cPanel or one of the database plugins but please do backup first for security. Here you can find the default value for the wp_user_roles option.
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https://wordpress.org/support/topic/reset-all-user-roles-to-default-wordpress/
How to change WordPress User Role capabilities – https://shinephp.com/how-to-change-wordpress-user-role-capabilities/