What is the equivalent of hook_user_view()?
hook_user_view() is still available in drupal 8, but the hook naming schema has changed, because the hook can now be used for all entity types:

https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Entity!entity.api.php/function/hook_ENTITY_TYPE_view/8.2.x

A more D8 way to do this could be a custom access check on the route:

https://www.drupal.org/node/2122195