But if you're just talking about development cost, we're just going back to the demand issue (which was what your original post talked about before it was deleted). Then again, I'm kind of sensing that you're also hinting at some prejudice against the technology itself by starting out to emphasize that RSS is a "legacy" technology?
I'm obviously not trying to attack you, but it does give the impression that it wouldn't matter if there was widespread demand for the feature or not; it's not going to be implemented regardless. Which would feel a bit unreasonable if true, but it'd be a stance you're entitled to take.
So in the end, is it about upvotes in this thread (since theoretically given enough demand, it would justify whatever development cost) or is there a clear stance that it won't be implemented, regardless of the demand for the feature?
That's kind of blowing up the situation. 😅
All you'd have to do is add a column in the user table (or any other low cost resource to retrieve data from to avoid changing the user table from being static data to dynamic data) that keeps track of the last accessed dtime, then add an endpoint specifically for the RSS links in the feed that will update the field and redirect to the selected manga chapter. The feed link itself would check the last updated field before doing the expensive call and reject it if it's past a certain period.
It's neither complicated nor prohibitively expensive to implement/test/maintain.