When a question is migrated from other sites, any tag that matches
^drupal-?\d+.*$ is removed. The problem is that Stack Overflow uses drupal-6, and drupal-7; as users on Stack Overflow are used to have those tags, they don't write in the question for which Drupal version they are interested in. (See https://stackoverflow.com/questions/13000925/link-a-content-types-field-to-the-user-profile as example.) As consequence, when one of those questions is migrated to Drupal Answers, that information is lost.
Rather than having those tags blocked, it would be better to allow tags like drupal-6, which are then made synonym of the actual tags being used on Drupal Answers (e.g. 6). If the version tag is not really necessary, because the question applies to any Drupal version, we will edit the question accordingly.
It is rather better to allow those tags than keeping to ask to the OP "For which Drupal version are you asking the question?" when the OP did originally shown for which Drupal version s/he was interested.
The black-list entry should be changed from
^drupal-?\d+.+$. In this way, tags like drupal-7 would be allowed, and set as synonym of the correct tag. Other tags, like drupal-7beta would not be allowed. As the latter is not used on Stack Overflow (the site from where most of the questions are migrated), it should not be a problem.