Hi! Not discouraging any discussion here, but just fyi, there have been a few posts on this topic, so you can check out some of the earlier answers (because, the TL;DR is that there's no "industry standard"):

• [META] Why is the restriction on current event posts set at 20 years? What makes that amount of time important? - /u/restricteddata explains the issue. Follow links to several discussion posts, not only on this subreddit's "20-year rule", but various examples from the field

• What is considered "History?" - a recent post featuring /u/CommodoreCoCo ; follow the link to another post containing a few more examples

• How old should an event be for it to be considered as "history" by the academics?

Most of these posts have been archived, so if you have follow-up questions for any of the users, ask them here, and tag their username to notify them