Assert the breadcrumb is, or is not, visible: the navigation landmark core labels "Breadcrumb" (nav[aria-labelledby="system-breadcrumb"]), or the .breadcrumb class of core's themes; the "breadcrumb" named selector overrides both. Core renders no breadcrumb where the trail would be empty, the front page being the usual case, which is what the negative form checks.
Examples
Then I should see the breadcrumb
And we should see the breadcrumbs
Then should see the breadcrumb trail
And I should not see the breadcrumb
Then we should not see the breadcrumbs
Pattern
Then(/^(I |we )*should( not)* see the breadcrumbs?(?: trail)?$/, …)
Part of @webship-js/drupal-cms-steps (github.com/webship/drupal-cms-steps), a step pack for Webship-js 2.0.x.