I am currently using Ubuntu 14.04.04 LTS Trusty Tahr. Ubuntu 16.04 LTS Xenia Xerus has been released but Software Updater so far has not notified me as it has other people, e.g.:
Note that I am talking about just LongTerm Support releases, not intermediate releases like 15.10, such as some questions address.
The Updater settings should be allowing this notification to occur:
When will the 16.04 notification occur?
(If the setting for new versions is changed to "For any upgrade", the Software Updater dialogue offers Ubuntu 15.10. Why?)