According to Windows Phone Arena, Viber updated their Windows 10 Mobile app today. Just last week, Viber announced that they decided to discontinue development for Viber on Windows 10 Mobile due to "limited company resources."
Now, it appears Viber has changed their mind, or maybe they're just pushing out one last update for old time's sake. The new Viber update brings the app version to 6.6.1, up from 6.6.0. Viber has made no official comment on whether or not it is abandoning Windows 10 app development and there is no changelog available for the recent update.
Stay tuned to OnMSFT and we will keep you updated on any new developments from Viber.
Thanks Peter for the tip!