if you only agree to gplus account and does things in google owned services, you are sharing everything without even knowing.
create a new account. agree to gplus. comment on youtube or use hangouts on device X. pronto, it was so frictionless that you didn't even notice. not it is public that you are using device X and that you commented on some youtube video.
what you say may be true for non-google services, which twitch does not fall under anymore.
When commenting on a YouTube video, there is a checkbox where you decide whether to share that comment on Google+ as well [0]. Your choice is sticky - if you uncheck the box and make a comment, then that setting will be remembered and it will never post on Google+ unless you re-enable it. And you can choose whether you want to share that comment with the public or with particular people [1] (I can't think of any other commenting platform that lets you choose specifically who you want to share your comments with on a public page).
Where are you seeing which device you are using shared on Google+? The only place I've seen something like that is if you have someone you've accepted as a Hangouts chat buddy and you've opted into the "share which device I'm on" in the Hangouts settings (all that shows though is if you're on a tablet, phone, or desktop computer though, not specifically which device).
yeah google employer, go check in your code repository history
1. how long my mentioned comment on you tube (for example) is tied to gplus
2. when that checkbox was added.
you will surely see its at least a six month difference or more. now some doesnt even have that option yet... but hopefully you will get the point with just example
create a new account. agree to gplus. comment on youtube or use hangouts on device X. pronto, it was so frictionless that you didn't even notice. not it is public that you are using device X and that you commented on some youtube video.
what you say may be true for non-google services, which twitch does not fall under anymore.