To quote u/crrpit from the last time this was brought up:

There is no 20-year rule in academia. Historians, and more broadly other researchers, are trusted to use their own discretion as to how far to bring contemporary ideas, politics and perspectives into their analysis - though, equally, they may find it difficult to convince their audience that their approach is justified if they take this too far. This means that a piece of writing which leans too far on contemporary politics might either be poorly received, or fail to pass peer review altogether. In practice, everyone draws their own line in these matters, based on their own research, beliefs about the nature of history writing and, inevitably, how they think it will affect their careers.

The 20-year rule here is in place not because historians are unable to process anything that recent, but because the purpose of this subreddit is to provide trusted information about the past. That mission is impaired if it becomes yet another forum for people to disagree about current politics in, or for asking historians to justify their political beliefs for them. In this sense, the rule is a blunt instrument, in place because it solves a practical problem rather than as a matter of absolute principle.

As you note, there is a historiography exception to the rule, to allow users to discuss the way that other historians have approached the past more recently than 1999. This rule is intended to facilitate normal historical discussion rather than provide a loophole through which politics can be introduced. As such, while there is obviously a grey area, it is enforced on that basis: if the post is likely (intentionally or not) to provoke political debate rather than facilitate historical discussion, it runs afoul of the exception. However, the goal is not to make users feel bad about breaking a rule, especially in good faith, but to preserve the functioning of the space we're all using.

We also made a point of this a few months ago when questions on the year 2000 became permissible. The gist is that it is an arbitrary distinct settled on because it provides enough distance from the event for an attempt to actually study and look at it objectively.

Its arbitrary, it's not any "official rule of academic history". It's a response to the tendency on the Internet for everything to become a political slanging match; twenty years is long enough ago to keep a lot of the noisiest contemporary political issues off the board.

So, for example, the internet is still drowning in 9/11 conspiracy nonsense-- if it were permitted here, we'd be drowning in it here, too. Once upon a time, there was similar conspiracy nonsense about Pearl Harbor and "Who Lost China" . . . these issues are now sufficiently antique that for most people the political valence is gone, and we can have a historical inquiry in these events without immediately descending into crankyland.

More generally, historians do approach contemporary events and can and do write about them at least somewhat informed by historical practice. The little known but very worthwhile publication, Current History, has just such an approach. Almost any analysis of contemporary international diplomacy and international law will be both historical and contemporary. If there's a full discussion of, say, US Naval transit operations near China today, it will include the 1998 Taiwan Straits incident and the Hainan P-3 Incident- and may well stretch back to Quemoy and Matsu. The same would be true of Brexit.

"Quemoy and Matsu" -- you'll likely say "what?" to that . . . but if you were a politically engaged observer of the 1960 election, it would have gotten you feverish . . . twenty years later, it was a footnote.

So twenty years, being roughly a generation, works reasonably well for an issue to have been removed from the irrational and intemperate of contemporary politics. Additionally, for an historian, it means that we have some sources beyond journalism to look at. . . for an historian to "do history", we generally want to be doing something more than footnoted journalism, we want kinds of data that wouldn't have been available at the time.

More philosophically, if you ask "what is history?" . . . the great historian Marc Bloch observed something to the effect that a historian is someone who writes about events that he never can witness. With that said, our earliest "historians" -- Thucydides, Herodotus-- were documenting events of their own lifetimes . . .

The first mention of the 20 year rule that I can find looking through web.archive.org is in a post by /u/artrw here from June 2012: http://www.reddit.com/r/AskHistorians/comments/ujf7c/meta_new_raskhistorians_official_policies/

Within that official rules post, the founder of /r/askhistorians says that:

History is typically define [sic] as 20 years old or older. Anything newer than this should be reported, and will be judged upon case by case.

At this point there are still only three mods - artrw, /u/eternalkerri and /u/agentdcf.

In this September 2012 post by /u/nmw: http://www.reddit.com/r/AskHistorians/comments/z9uyg/meta_widescale_revisions_to_the_official_rules/ about the rules, the rules are starting to look more like their modern form. This seems to be the first lengthy official set of rules (but which portrays the rules it’s presenting as basically making official what had been unofficial practice until then.)

The text in there related to the 20 year rule reads:

For the purpose of discouraging too much speculation about current events, we request that users in /r/askhistorians confine themselves to questions about events taking place prior to 1992. This twenty-year window is not without its complications, but we wish to keep the comments in /r/askhistorians focused on events that have already had a chance to become more or less settled.

Anything focused on events after 1992 should be reported to the moderating team, and will be judged on a case-by-case basis.