Missing indices status after upgrading to CiviCRM 4.7.21 on WordPress 4.8
This problem has been raised before and you can view the thread at How to find and/or fix missing indices?

There is also an entry in JIRA at the following link https://issues.civicrm.org/jira/projects/CRM/issues/CRM-20533?filter=allissues

Hope this helps.