I just realized I used terrible syntax in the text you quoted. I apologize for that. I know AD&D 1e and 2e had PHB because I played them extensively. What I meant was that BECMI and OD&D were not systems I ever played (not in their heyday anyway; I have since played a few pick-up or one-shot games). And to your point about the 2e optional rules it was my experience not only that most players expected they were in play unless the DM said otherwise, but that most DMs did, in fact, use them as well. [MENTION=22779]Hussar[/MENTION] pointed this out some of the reasons why quite well; the so-called "optional" rules were used in DM tools like the MM and adventure modules which quite a few players had encountered, and this reinforced the idea that these rules were only optional if the DM said he was not using them.
I went back over the holidays and re-read the section on these optional rules. There are two very brief paragraphs in the 5e PH in Chapter 6 the say (paraphrasing) that the DM must allow these rules. The rest of the chapter (which, let us be honest, is likely used as a reference tool by most players rather than a "read from front to back") does not really mention the DM at all. For players converting to 5e or coming back to D&D from 3e or later, they will likely look at these rules and say "ok, so they kept feats," and assume that is all there is to it.
IMHO, WotC dropped the ball on this one. They should have called this chapter "Optional Rules Modules," it should have been larger, and it should have included all of those little "optional" sidebars found throughout the PH, e.g. the optional subrace options. Furthermore, they really ought to have included chapter titles in the margins on ALL pages, not just for easier reference (this is a big complaint I have with the PH, but that is another discussion), but to remind players each time they visit this chapter that all of these rules are indeed optional. As written, the WotC writer usage of the term "optional" in the PH reminds me of the Pirate's Code from Pirates of the Caribbean. "They're really more like guidelines."
Sidebar: I played 5e a couple times with friends from my hometown over Christmas vacation, and to no surprise of mine it appeared we were all under the assumption that the Chapter 6 rules, including multiclassing, were allowed unless otherwise stated. I created a human cleric and chose the option of +1/+1 stat bonus, skill bonus, and feat, and the DM didn't say a thing. When one player leveled up his 2nd-level fighter to 3rd, he said "my next level will be wizard," and the DM didn't say a thing. I also noted that in Adventure League play (although I realize the majority of D&D groups are most likely NOT Adventure League games) these rules are being used as well. Rather the Adventure League rules tend to explain which optional rules are NOT in use for Adventure League play.
But I think I have beaten this dead horse enough. That is my opinion, my experience, and that is how I intend to proceed. As I have said from the beginning, YMMV. Your experience and opinion of the rules may differ greatly from my own, and it may even be a regional difference in the perception of the rules. Whatever the case, it is not really worth arguing further about. Feel free to make your closing arguments. That was mine.