Is 20 years an industry standard, and if not where did the 20 year rule come from, and what is the earliest and most recent points that professional historians would mark as the difference between current events and history?
The reason for the current events rule is explained pretty well in this Rules Roundtable; briefly, the goal is to allow discussion of events once there's been time for their effects to settle and some scholarship removed from the immediate event to be written. The number of years is arbitrary, but most universities have some sort of a 10-30 year rule after which history becomes political science, or contemporary studies, or some other term. (Because this is Reddit, we also find that the 20-year cutoff insulates us pretty well from political bickering, and discourages people from replying using their own personal anecdotes, which we disallow anyhow.)