In the case of DLC, I think it is better to update the old answer with an additional section. Most DLCs are not substantive enough to get their own tags like Iceborne, since most of them only add a few extra hours of content. There is no issue with using the Iceborne tag to ask an Iceborne specific question but in this specific case, it is best to update the existing answer with the DLC information.
Patches, on the other hand, are a much trickier situation. Games like Minecraft (sandbox games) or Warcraft 3 have patch specific info that is useful at all times since it is occasionally desirable by users to play on a specific patch. If the patch info is not included in the post and another question is asked with version specific information required, I think both questions should remain open and all answers should be preserved. It is my personal opinion that this information is equally useful but the general community may feel otherwise.
On the issue of reopening old questions, I do not feel that reopened questions get as much attention as newer questions. I also believe that adding additional answers to already answered questions every time a patch hits would make certain questions too cluttered. Most users probably will not scroll past the accepted answer and will only get the outdated information. It is for this reason, the community has to decide if they want to maintain historical patch data which comes with more cluttered answers or additional questions opened on the same topic, or removing historical data which leaves us with fewer questions and less clutter but removes an excellent resource for hard to find data.
The most optimal solution, in my opinion, is an added tab in questions for questions that have completely outdated information. There are questions that have info that is completely inapplicable to the current game and will never be applicable again. A good example of this kind of question is something like this CSGO Case Drop Rates. Currently, I find I can only get 1 case drop per week and even if you did find a way to play an old patch, you probably would be unable to get drops since they are so closely tied with steam accounts. If you look through the linked and related questions, there are many outdated answers still open with info from previous operations, patches, etc.
With this feature, the community would have to vote a question into this category and approved by a high rep user. The current answer would be left behind and maybe given an edit to mark that it was the previous answer. This way you could get standard rep for answering a question, draw attention to outdated questions better, removes clutter/additional questions, and generally, incentivizes users to interact with old questions.