RetroRoleplaying has an excellent post here in which Randall points out an example of a 1980 article on variant Dungeons & Dragons combat systems which presents an ascending AC system that’s more or less exactly the same as the one in 3rd Edition D&D onwards.
The point isn’t to accuse the 3E designers of plagiarism or anything like that; the point of ascending AC is to flip the direction of the AC progression but keeps the underlying maths precisely the same in order to make things more intuitive in play, and unless you fiddle with the values of different armours there’s really not much design freedom in how you implement that. The point, however, is that ascending-vs-descending AC – along with all sorts of other shibboleths – have become latched onto by some of the old-school revivalists as being Real D&D, and if you dispense of them it’s no longer Real D&D. If anything, as Randall points out the spirit of Real D&D – if you are defining that as “D&D as it was played in the early days” – is one of massive experimentation and gleeful fiddling with every aspect of the system, with variant rules cropping up at more or less every table.
That isn’t to say that I intend to imminently switch to a spell point system in my own Advanced Dungeons & Dragons campaign (Vancian magic has a particular flavour to it which I like, and which the people who are playing spellcasting classes in the campaign knew they were getting when they signed up), or to ascending AC (I find it faster just to get the players to roll D20s and have me say whether it’s a success or failure over Roll20; I kind of like having a bit of mystery as to where the to-hit target is, though if the players ran into a monster they would have serious trouble actually hitting I’d certainly make sure to warn them of that). But it is a useful reminder that it isn’t screwing around with the system which stops a game being like D&D; if anything is capable of doing that, it’d be screwing around with the basic principles of how people interact with the game (the dialogic, refereed, anything-can-be-attempted, miniatures-optional model identified in Playing at the World).