Programmatically / by Rules, remove reference of a deleted user from contents created by that user.
supposing the content you want to reassign is of node type, here is how to do it programatically (in a hook_update_N() per example or /devel/php or drush ev...)

  $deleted_uid = 42;
  $reassigned_uid = 80;
  // Reassign nodes (current revisions).
  module_load_include('inc', 'node', 'node.admin');
  $nodes = \Drupal::entityQuery('node')
    -&gt;condition('uid', $deleted_uid)
    -&gt;execute();
  node_mass_update($nodes, ['uid' =&gt; $reassigned_uid], NULL, TRUE);
  // Reassign old revisions.
  db_update('node_field_revision')
    -&gt;fields(['uid' =&gt; $reassigned_uid])
    -&gt;condition('uid', $deleted_uid)
    -&gt;execute();


hope this helps