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use Drupal\workflows\WorkflowManager;
use Drupal\Core\Entity\EntityInterface;

// Assuming you have the entity and its workflow ID.
/** @var \Drupal\Core\Entity\EntityInterface $entity */
$entity = ...;
$workflow_id = 'your_workflow_id';

// Get the workflow manager service.
$workflow_manager = \Drupal::service('workflows.manager');
$workflow = $workflow_manager->getWorkflowById($workflow_id);

// Get the current state.
$current_state = $workflow->getState($entity);

// Get available transitions from the current state.
$transitions = $current_state->getTransitions();

// Collect roles associated with these transitions.
$roles_involved = [];
foreach ($transitions as $transition) {
  foreach ($transition->get('to_states') as $state_id) {
    $state = $workflow->getState($state_id);
    $roles_involved += $state->get('roles');
  }
}

// Now $roles_involved contains the roles associated with the next possible transitions.