I don't think that analysis stands very well. The introductory section of the 2e Players Handbook refers to itself, the DMG, and first volume of the Monstrous Compendium as essential. Everything else is called optional. How is that really different from 3e's core/not core split? I think perhaps the concept of core/not core may have become more defined throughout the tenure of 2e (probably because of people's experiences with the Complete Handbook series) until it reached an explicit "Core Rulebook" label for 3e, but the idea was clearly there in 2e's intro through the labeling of some books essential and others optional.
It's kinda funny how 4e gets hammered for not including monsters in the 1st Monster Manual, but, 2e apparently gets the pass. By your definition, the following monsters are no longer core: Basilisks, dopplegangers, dryads, dwarves, drow (!), ettin, gargoyles, hippogriff, lamia, mimics, otyugh, roc, rust monster, and troglodytes (I'm only listing very, very common monsters).
Yet, I've seen all sorts of modules that assume that those monsters are in play and the DM has access to those monsters.
From the early pages of the 1st edition Unearthed Arcana:
1e Unearthed Arcana Page 4 said:
In this book, those experiments are made concrete. The experiments are completed. The suggested rules are now official and final.
What I don't find? Any mention at all that these rules are optional. These are official rules. They are meant to be used and future publications in AD&D, I know, use these rules. You see comeliness and the Barbarian class in later published modules for example.
Now, to be fair, buried on the back page of the 2e Complete fighter's Handbook, on the back cover, is the only example of "optional supplement" that I could find, but, it is there. They do, in the back cover blurb, say that this book is an optional supplement. Certainly nothing between the covers suggests that this isn't core. And, considering that optional rules were part of core in 2e (proficiencies were optional after all - does that mean they aren't core?), I'm inclined to agree with [MENTION=44640]bill[/MENTION]91 that this might have been something that developed over time.
For example, on a personal level, no one I knew considered the Skills and Powers books to be core, but rather a parallel but different game. But, on a purely personal level, groups that I played with assumed the Complete Books were in play all the time. Some of the later splats, you might have asked, but, if a new player showed up with a PC with a kit from, say, The Complete Fighter, no one would have raised an eye. And that was when I was playing in the university game club where you had gamers from all over the country. Confirmation bias and all that, but, I remember that the Core trinity was a big shift when 3e came out. It certainly wasn't something I saw assumed prior to 3e.
/edit Shoot. Ninja's by [MENTION=3586]MerricB[/MENTION]