In my opinion the most accurate thing to do in this cases is to answer those outdated questions with an updated answer, trying to leave some help for anyone still interested in the topic specifying that at the time the question was made it was 100% valid. For example, to answer the question you used, we could answer something like this:
Diamond Arrows and Spectral Bolts do not expire anymore. This question was made when this items were expirable but there has been some updates to the game and they no longer expire. In fact, you don't even need to create them anymore, you can just buy them from NPCs and they stay for as long as you want in your inventory.
This way the question is still valid and helpful because we are informing that the original question is outdated but we are helping with new information related to the same question. This is extremely useful for people who use code words such as "diamond arrow" to search for information, because those outdated questions will obviously appear for them and they can still be of help.