Is there a "registry rebuild" command? I just did a fresh Drush installation, and now I have version 8.1.15.

When I run drush rr, I get the following error message. 


  The drush command 'rr' could not be found.  Run drush cache-clear drush to clear the commandfile cache if you have installed new extensions.


Even after running drush cache-clear, I still get that rr is not a known command.

Is this still available in Drush 8?
Take a look at https://www.drupal.org/project/registry_rebuild

The documentation is there:


  How To Use Registry Rebuild With Drush
  
  This is the preferred technique, and it should work with multisite installs.
  
  You can just drush @none dl registry_rebuild-7.x and drush will download it into your .drush folder. (Alternately, you can obtain the package another way and copy the folder into .drush yourself.)
  
  Make a backup of your database.
  
  Clear Drush own cache with drush cc drush
  
  On a multisite install, either use correct drush alias, or cd into the site you're rebuilding, as in cd sites/mymultisite
  
  Run drush rr or drush @sitename rr for soft rebuild (default), or:
      To force aggressive rebuild from scratch run drush @sitename rr --fire-bazooka