GreXXL I'm not sure Who Read solves any of the concerns raised on this discussion.
A built-in "solution" if you are looking for new discussions is to change the sorting from "Latest" to "Newest". Then everything above the last discussion you remember reading will be new.
If your community has a need to follow new discussions as they appear, maybe you could benefit from Follow Tags https://discuss.flarum.org/d/20525-friendsofflarum-follow-tags Users who want could subscribe to get an email on new discussions.
Showing a visual indicator next to a discussion if it it has never been read at all should be very easy to do with a custom extension.
Showing an indicator in the tags or categories page is a bit more complex, it would require saving the date or ID of the last discussion read by every user under this tag as well as the same value for the newest discussion in every tag, so that the values can be compared at run time. Flarum already memorizes the last discussion with activity, but this cannot be re-used because that's based on the last reply, not whether the discussion is new.