hook_views_plugins_alter() replaced by plugin type-specific hook_views_plugins_PLUGIN_TYPE_alter()

The new hook to use in order to hook up a new plugin to a display is
hook_views_plugins_display_alter(array &$plugins)
and set the the new class accordingly. I.e.
$plugins['page']['class'] = 'Drupal\my_module\Plugin\views\MyExtendedPage';