How to create revision for a node programmatically
// Make this change a new revision
  $node->setNewRevision(TRUE);
  $node->revision_log = 'Created revision for node' . $nid;
  $node->setRevisionCreationTime(REQUEST_TIME);
  $node->setRevisionUserId($user_id);
  $node->save();


Use Above Code to create a revision. :)