8.4.4 CKEditor Link & Unlink Buttons suddenly missing
I figured out the issue.  Apparently something changed in the Drupal 8.4.4 update that caused the scripts related to the link/unlink buttons to work longer than usual, which was causing a PHP timeout exception (when checking the browser console it gave an HTTP Result Code: 200 error, which apparently happens regularly with Drupal).  So in my settings.php file I increased the timeout by adding the line:

ini_set('MAX_EXECUTION_TIME', 7200);

You can also configure this in your host settings.