The only 'announcement' from Google was a post by a Google employee (Nikhyl Singhal):
https://plus.google.com/+NikhylSinghal/posts/MjyncJEbzxKThere was no mention whatsoever of Google Voice XMPP changing. May 15, 2014 was the date third-party application
developers (such as Obihai) had to stop making unauthorized use of Google Voice:
"There are a few third-party applications that provide calling and SMS services by making unauthorized use of Google Voice. These apps violate our Terms of Service and pose a threat to your security, so we're notifying these app developers that they must stop making unauthorized use of Google Voice to run their services and transition users by May 15, 2014."
If you're aware of any announcement
from Google stating that Google Voice XMPP will stop functioning on May 15, 2014 or that Google has any plans to disable it on any date, please post a link to it.