For what it's worth, AOL had this 25 years ago and never even had complaints. And yes, people came to realize that "DanBlake read this" meant DanBlake's client

For what it's worth, AOL had this 25 years ago and never even had complaints. And yes, people came to realize that "DanBlake read this" meant DanBlake's client had read this, and you, personally, might not have seen it yet - or, if you had, you'd marked it unread for later reading. But we had that feature from day 1; user expectations grew up around it. Introducing this kind of behavior mid-game is dangerous for all the reasons everyone's mentioned.