It could be argued - which I did - that the XMPP connectivity to Google Voice was legitimate because of the clear public documentation of the Google Talk service, which included Jingle. The only missing piece from that documentation was JID format for outbound calls (easily discovered).
GVSIP was a 100% discovery effort, which is what I believe triggered the Google folks to sound the warning about reverse engineering.
The point is - echoing kdthomas a bit - there was legitimate connectivity from Asterisk to Google Voice (though never offered by Google officially as a "service" if you look through their descriptions of the GV product) but that ended around July 2018.
I look at it differently. We're not doing anything TO their service, it's only a question of what we do WITH their service. It's only a matter is which telephone device we use to access Google Voice.
Regardless of how we access their service it doesn't have any effect on how Google Voice operates. Whether we access with an Obihai device, their web interface, Hangouts, or even Asterisk, it makes no difference on Google's end. Nothing is added, removed, or changed with regard to Google Voice service.
What is becoming apparent is that they're getting something from individual users that they're not getting from group users accessing Google Voice with Asterisk. It would seem that google gets marketing information by knowing who we call and who calls us. While that may sound creepy on the surface, keep in mind that traditional phone services also had that same information. They may not have sold or used that information, but they certainly had it.
But it's a lot more difficult to market products to a large group of people sharing a few phone lines. There's no telling what the individuals are interested in, or even how to contact them. With individual accounts Google knows who you are and what your email address is.
Whatever it is that Google gets, they're not getting it from Asterisk users. Discouraging Asterisk users from using Google Voice probably lowers google's cost pf providing Google Voice service. I think that's what's behind the recent changes.