By that measure, I doubt we get much agreement on what constitutes rules-light in a TTRPG without describing the actual mechanic.
With the
possible exception of the simplest versions of Basic, genuinely zero versions of D&D or its direct descendants (like PF) are actually rules-light games. Some editions may be lighter than others, but genuinely not one of them is a rules-light game. As others have said, ultralight games exist that are literally one page, sometimes less. I ran a DW game for several years (before I started adding supplements) where the entirety of the rules-text my players needed to know--
including unused character playbooks--fit into a 26-page PDF.
By comparison to literally any clearly rules-light game, essentially every version of "D&D" ever published is an
ultraheavy game.
The
only way you could argue that any version of AD&D was "rules-light" is that so many people ignored or completely missed so many of the game's rules that it was
effectively true that most people used no more than 90% of the actual game, and many many people used less than 50%. But "we didn't use half of the rules text due to ignorance, apathy, or avoidance" is not in any way a claim that
the game itself is rules-light; it is simply that many people, whether accidentally or intentionally, ripped out huge chunks of the game and just did not use them.
There's also what do you want to do with a rules-light game - I can play a one shot game using a Jenga tower. I wouldn't want to play a campaign using that as a mechanic, for instance.
Sure. But no one in their right mind would argue that Dungeon World is of comparable "weight" to AD&D1e. AD&D1e is a
behemoth compared to DW. It's just patently ridiculous. Maybe--
maybe--you could argue that AD&D1e was a "early installment weirdness"
very patchy effort at rules-medium? But huge chunks of AD&D1e's clunky, weird, baroque rules got ossified into Eternal Tradition, rather than being (as they were in most cases) pure ad-hoc, and occasionally off-the-cuff, solutions to problems as they cropped up.
Which, incidentally, that is another great reason why AD&D (1e or 2e) is not and never was a rules-light game. Massive amounts of its rules-text are scattershot
stuff Gygax invented for his home game that then became laid down as mandatory expectations. That's why we have a Van Helsing vampire hunter Cleric. That's why we have the arcane/divine magic divide. That's why you had the absolutely impenetrable "saving throw" categories. Etc., etc. Now, don't get me wrong, Gygax had some very savvy ideas about game design and there's quite a bit of cleverness in the rules, particularly given this was the bleeding edge of game design at the time. But to call the grotesque melange of (almost always) patternless rules "light" in any way or form is either bad comedy or worse logic.