Parts of it from the 1e PH are mostly fine.It was fine in the AD&D PHB. It’s only when you try to use every caveat and Dragon article and later supplement that it becomes insane.
It is not labeled as such.Weapon type vs armor was an optional rule.
"Everything in the ADVANCED DUNGEONS 8 DRAGONS system has purpose; most of what is found herein is essential to the campaign, and those sections which are not - such as subclasses of characters, psionics, and similar material - are clearly labeled as optional for inclusion."
"Thus, besides the systems, I have made every effort to give the reasoning and justification for the game. Of course the ultimate reason and justification is a playable and interesting game, and how much rationalization can actually go into a fantasy game? There is some, at least, as you will see, for if the game is fantasy, there is a basis for much of what is contained herein, even though it be firmly grounded on worlds of make-believe. And while there are no optionals for the major systems of ADVANCED D&D (for uniformity of rules and procedures from game to game, campaign to campaign, is stressed), there are plenty of areas where your own creativity and imagination are not bounded by the parameters of the game system. These are sections where only a few hints and suggestions are given, and the rest left to the DM."
True but there are references in other places to this set of rules that indicate they are optional.It is not labeled as such.
<snip>
1e DMG page 28:It is not labeled as such.
From Gary's preface to the PH:
And from his introduction to the DMG:
AD&D 1E had very few significant optional rules. In contrast to 2E, which had a lot of them.
I think the PH just has Bards and Psionics as optional.
The DMG has an optional suggestion on page 25 that you can give players extra starting items if it's a difficult campaign, but that's not really a rule. The optional rules in the DMG are mostly tiny little tweaks, rather than major systems, as Gary alludes.
There's an optional rule for large shields giving +2 against missiles (p 28), optional parameters for Cure Blindness (p 41), +3 or greater weapons and armor may optionally function on other planes (Astral Spell, p 42), optional restriction on the Enlarge spell (p 44), one for shorter scatter distance for grenade-like missiles at short range (p 64), an optional rule for mixed groups of undead and Turning (p 65), the Special Note Regarding Fighters' Progression to smooth their attack advancement (p 74), an optional rule allowing unconsciousness at between -1 and -3 HP instead of the official rule for unconsciousness only at exactly 0 HP (p 82), Fighters being optionally allowed to take weapon proficiency with siege engines (p 109), optional double rate of fire rule for Boot Hill firearms and optional save vs dynamite explosions but increased base damage for it ( p 113), optional charge limit for certain powerful magic rings (p 122 & 129), an optional rule that 1% of wands may be trapped to backfire (p 135), the Mirror of Mental Prowess may optionally allow planar travel (p 150), deities who create or control an artifact may be able to reverse the effects of some artifact powers (p 156), scimitars may optionally use a table for properties instead of just defaulting to a single +2 option (p 169), cursed backbiter spears may appear as +2 or +3 instead of just +1 (p 169 as well), random lower planes creatures may optionally have poisoned tails or mouths (p 195).
As for more major systems, the DMG offers four different alternatives for ability score generation, but they aren't presented as one official way and three optional variants, but as all being official and recommended.
Thanks for reminding me! I'm slapping my forehead on this one. I used "opt" as my search term in the texts.1e DMG page 28:
WEAPON TYPES, “TO HIT” ADJUSTMENT NOTE
If you allow weapon type adjustments in your campaign please be certain to remember that these adjustments are for weapons versus specific types of armor, not necessarily against actual armor class. In most cases, monsters not wearing armor will not have any weapon type adjustment allowed, as monster armor class in such cases pertains to the size, shape, agility, speed, and/or magical nature of the creature. Not excluded from this, for example, would be an iron golem. However, monsters with horny or bony armor might be classed as plate mail if you so decide, but do so on a case-by-case basis. Naturally, monsters wearing armor will be subject to weapon type “to hit” adjustment.
"If you allow" reads to me here as optional and discretionary.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.