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https://blog.discourse.org/2021/04/standing-on-the-shoulders-of-a-giant-elephant/
#Discourse (a piece of software used to run online forums) appears to not only track what posts someone views, they track the amount of time they spend viewing them. This doesn't sound good for #privacy?
The UI / UX is fairly similar to NodeBB which we've seen around.
https://nodebb.org/blog/nodebb-1-9-0-worlds-first-gdpr-compliant-forum-software/ This is a promising sign, although I haven't looked far into it.
While it might be useful to keep track of how far someone has read in a thread, so as to allow them to pick up where they left off, I don't think there is a good reason for storing every post someone has read, and especially, not how much time they've spent reading each and every one.