That works in WhatsApp as each user has a single device. Messages are only delivered into an app, and besides skimming the notification previews you can’t “read” a message without opening it.
Discourse OTOH is all about users accessing from multiple endpoints, some of the ways to consume such as email couldn’t convey a read state if you wanted to.
I wouldn’t mind a “read up until” notification for Messages, but it would need a huge disclaimer that it’s not an infallible measure.