There is a mailinglist for Flask hosted on python.org that you can use for both user requests and development discussions that may not fit well in a Github issue or StackOverflow question.
You can subscribe or change your subscription options at https://mail.python.org/mailman/listinfo/flask/. If you're having problems, don't forget to check your Spam folder.
You can view the list archives at http://mail.python.org/pipermail/flask/.
Before July 2015, we used librelist for the mailing list. You can view those archives here: /mailinglist/archive/. Just keep in mind any new messages should now be posted to the python.org Flask mailing list mentioned above.