I would be inclined to argue against keeping track of "last seen" times; and even more so against publicly dispaying it.
From simply a legal perspective, there's the issue of the GDPR's requirement for data minimization. (See for example the GDPR, article 5 paragraphs 1(b), 1(c).) While one could argue that keeping some kind of "seen when" data for some reasonable amount of time serves a legitimate purpose for not-publicly-accessible logs, it seems more dubious that publicly displaying that data serves a legitimate purpose.
From an individual user's perspective, there's the issue that publicly displaying (and yes, to signed-in users only still counts as public when anyone can get an account with no significant barriers to access) the time when a user was last seen on the site constitutes an information leak about the person associated with that user account. Yes, it's a small leak, but it's still something that should be weighed against the benefit. If the benefit of what's being proposed is small, then IMO even a small information leak can be a significant mark against that proposal.
You mention the case of "deciding when to return to a post" as a benefit of displaying a "last seen" time. I don't really see the benefit here; certainly I tend to mainly keep track of what's going on on the front page, and visit posts that I'm interested in; if there's nothing going on, and I'm not particularly interested in the subject, then there's likely little reason for me to visit a post, whether or not the user who posted it has been "seen" (for some definition thereof) since some particular point in time. I fail to see how determining which user posted something, then check their profile, would improve the "decide whether to visit a post" workflow.