What button types are available in Drupal 8?

According to this issue the only button types are ‘primary’ and ‘danger’ (plus the standard default, which doesn’t need nor get any extra class).

It references the proposed Seven admin theme style guide but that’s hard to count as real documentation.

It’s not documented anywhere here - https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Render!Element!Button.php/class/Button/8.9.x - except in the source, which reveals that it will add a button--whatever class to the button if you were to put in ‘whatever’ as the button type— that is, there aren’t any button types in core, except for the conventions that themes adopt.

But the conventions that themes adopt for button types should be documented!