They introduced that in ... Hangouts 5, or was it 6? I found it more than aggravating since Google had been pushing Hangouts as the replacement for Messaging (Android SMS/MMS app in 5.x and earlier), and promised integration with Google Voice.
I've been using
Google Messenger on 5.1.1 on my Nexus 4 for my SMS needs, for the past few months; it seems to be an update to
Messaging with some new usability and functionality. I've stayed on Hangouts 4.2 (the last version with a working widget), using it only occasionally for G+ chats. Both have great widgets (Messenger provides a person-specific widget that you can put on its own screen).
Trying Hangouts again in my "new" Nexus 5 (running Marshmallow on a phone for the first time), I'm seeing the GV integration there -- yay[1] -- but they STILL haven't restored the widget (as per your screenshot). Even more upsetting, when I switched SMS over to Hangouts yesterday, Hangouts told me this (pic)!
[1] When I went to the usual Google Voice web page for handling texts, it directed me to Hangouts to continue chatting, and sure enough, SMS is handled in Hangouts.google.com. Nice.