I would say the “this portion is administered by KKL” tagging should be kept if it is present and accurate, and removed otherwise.
Rationale: 1) the map should have no incorrect data, 2) the extra information is conceivably interesting to some data consumers. By analogy, if KKL were a municipality and the forest was a built-up area, we’d map the city limits even though there are buildings both inside and outside those limits, won’t we?
My 2 cents, based on reasonable guesses:
In my opinion, it is unlikely to be used except for very specific and extremely obscure cases. Furthermore, someone who wants such specialized data likely knows exactly what they’re doing, and can obtain it directly from KKL, where it’d be more up to date. The OSM data is 5 years old.
Sure, if the data is out of date, then either tag it appropriately or delete it, and let interested parties fetch it from the edit history or from KKL. (Sorry, didn’t see your answer before.)
I think people have been editing these relations on the assumption that they represent physical forests. Nobody should use them for KKL administration data - they will likely be very inaccurate now.