This is a nice move by Google: if you disable resharing on a post, it also disables the ability for users to add +mentions to the post. It wasn’t a big issue, and it was one that Google debated internally for a while, but they ultimately decided to block it for the sake of user privacy.
Many of you have asked for this, and now it’s here!
Now, when you disable resharing (from the arrow at the top corner of your post), it also disables +mentions. That way, you can really clamp down on who’s allowed to see the post.
There were reasons why we originally allowed +mentions even when sharing was disabled, but your feedback convinced our team to change that. Keep sending feedback! We’re listening.
Let me tell you, sometimes it’s hard to answer your feature requests without giving away the fact that we’re already working on them. =)