Custom restrictions using hooks (for developers)

Site-specific restrictions not requiring UI-based changes can be done by implementing hook_plugin_filter_TYPE__CONSUMER_alter() (which is invoked for both themes and modules). Examples follow.

Restrict certain blocks across all Layout Builder usage sitewide

/**
 * Implements hook_plugin_filter_TYPE__CONSUMER_alter().
 */
function mymodule_plugin_filter_block__layout_builder_alter(array &$definitions) {
  // Explicitly remove the "Help" blocks from the list.
  unset($definitions['help_block']);

  // Explicitly remove the "Sticky at top of lists field_block".
  $disallowed_fields = [
    'sticky',
  ];

  foreach ($definitions as $plugin_id => $definition) {
    // Field block IDs are in the form 'field_block:{entity}:{bundle}:{name}',
    // for example 'field_block:node:article:revision_timestamp'.
    preg_match('/field_block:.*:.*:(.*)/', $plugin_id, $parts);
    if (isset($parts[1]) && in_array($parts[1], $disallowed_fields, TRUE)) {
      // Unset any field blocks that match our predefined list.
      unset($definitions[$plugin_id]);
    }
  }
}

Restrict access to layouts and blocks by user role

/**
 * Implements hook_plugin_filter_TYPE__CONSUMER_alter().
 */
function mymodule_plugin_filter_layout__layout_builder_alter(array &$definitions, array $extra) {
  $currentUser = User::load(\Drupal::currentUser()->id());
  if (isset($definitions['diagonal']) && !$currentUser->hasRole('administrator')) {
    unset($definitions['diagonal']);
  }
}

/**
 * Implements hook_plugin_filter_TYPE__CONSUMER_alter().
 */
function mymodule_plugin_filter_block__layout_builder_alter(array &$definitions, array $extra) {
  $currentUser = User::load(\Drupal::currentUser()->id());
  if (!$currentUser->hasRole('administrator')) {
    unset($definitions['inline_block:icon_with_text']);
    unset($definitions['inline_block:news_cards_external']);
    unset($definitions['inline_block:program_list']);
    unset($definitions['inline_block:profile_collection']);
    unset($definitions['inline_block:view']);
    unset($definitions['inline_block:work_study_list']);
    unset($definitions['inline_block:advanced_accordion']);
  }
}