It would definitely be possible, as a plugin. You would need to sacrifice some features for that (or make them work less well), and you would need to consider in relatively great detail where functionality stops and tracking starts (for example, if you remove the user-last-seen-stats you would still have the latest timestamp of a users posts and likes as a minimum last-seen-date).
I think the better bet would be to go the consent and contract legal basis routes where users know and agree that the data that is collected about them (exclusively) serves the goal of offering them a good forum experience. You even won’t need to go the legitimate interest route since you’re not selling their data to advertisers or anything.