Hi OP. There have been many threads on this topic - some announcements from the mods, others meta questions like yours. Take a skim through these - if you have specific followup questions, ask away.

• In this sub - Rules Roundtable #5: The Current Events/Modern Politics/"20 Year" Rule
• A question - [META] The 20 year rule on post comments should be atleast partially waived if the main question is answered.
• By historians generally - AskHistorians uses a 20-year rule to consider something to be "history." Is this a common rule among historians, or is it just a pragmatic measure to moderate this community? If so, how do scholars decide when something can be called "history"?