Wizards of the Coast has never explicitly stated what they built into their designs for 3.5e; it’s only apparent from the progression of monster stats as one progresses from one CR to the next. And since CR is wildly inconsistent, that is not an easy thing to do.
That said, while Wizards of the Coast never delineated their process to the point that they said that they built in certain items, we do have some hints. Magic Item Compendium refers to “must-have” item effects, acknowledging that some items are not to be skipped.
One of the most frustrating roadblocks to using interesting, unusual magic items is that they take up body slots that you need for an ability-boosting item (such as gauntlets of ogre power), a ring of protection, or another must-have item. To address this issue, Magic Item Compendium presents official rules for adding common item effects to existing items.
(Magic Item Compendium, pg. 233)
If they know—or, at least, believe—that players will not skip these effects, it stands to reason that this influences their monster design.
The particular effects that they list in this section are as follows:
- Deflection bonus to AC from body, ring, or shoulder slots
- Armor bonus to AC from arm or body slots
- Enhancement to natural armor bonus to AC from body or torso slots
- Enhancement bonus to each ability score, from various body slots
- Resistance to energy in body, ring, shoulder, or torso slots
- Resistance bonus to all saving throws, in shoulder or torso slots
(from Table 6–11: Adding/Improving Common Item Effects, Magic Item Compendium pg. 234)
Moreover, it’s almost-certain that expectations regarding the magic on warriors’ weapons was also considered in the design of encounters. Certainly, the prevalence of damage reduction requiring particular weapons increases significantly with CR.
It may be worth noting at this point that Pathfinder has much the same concept, in the “Big 6” “mandatory” magic items, five of which provide bonuses from MIC’s list (energy resistance excepted), and the sixth of which is that magic weapon I mentioned. I don’t know if Paizo has ever commented on or codified the “Big 6” but that concept is certainly widely found in Pathfinder-playing circles.
Despite all this, however, there is also a question of how much Wizards of the Coast understood the nature of their own game, which leads to questions about how much of the things players talk about as mandatory were expected or intended to be so, and how much of it is emergent behavior reacting to the overall rules and players’ experiences. For example, there are all kinds of things that require magic to interact with, which for non-magic classes means magic items. Was Wizards of the Coast aware of the prevalence of these, and their impact on the game? It’s not entirely clear. If you want to interact with a lot of content, you are going to need answers to things like flight and teleportation, illusions, fear and mind-control, and so on. There exist answers to these in item form, in generally-increasing reliability as one pays more gold for them. Was it understood and expected for players to carefully purchase items to ensure “coverage” for these situations, or was it expected that there would be situations in which a player character simply had no answer and could not contribute directly in that instance—and that was OK? We don’t really know. Certainly, most players seem to prefer to avoid that, hence the various guides, handbooks, and questions here about how to get coverage and keep a character contributing.
The often-unspoken assumption that players make when they tell other players that certain items are “mandatory” is that their audience consists of players who want their characters to continue to be able to act and contribute to the challenges that they face—that this is, in effect, what it means to “play” the game, and that players want to play.
But there might be evidence that Wizards of the Coast didn’t get that. For example, nearly half of Table 6–11 referenced above refers to a way to get more AC, and indeed, you need all those things in order for your AC to stay at a point where monsters have a reasonable chance of missing you. The problem is, they cost a ton—all-but-ensuring you won’t have good “coverage” of all the special cases an adventurer needs to deal with. And that’s just to get “decent” AC. As a result, by mid levels, the overwhelming majority of characters wind up kind of just giving up on AC—it costs too much to get enough to matter, so characters often just focus on other defenses to protect them from attacks and more. So while these AC bonuses do very much seem to be “expected” by the game’s monster design, players’ responses seem to have not been. The quoted section of Magic Item Compendium refers to a ring of protection as “must-have,” but really a whole lot of characters will sell one if they find one, particularly for higher deflection bonuses, as they are quite expensive.
And since you bring it up in the question, it should be mentioned that Vow of Poverty is particularly strong evidence that Wizards of the Coast had no idea what items were most important, since that feat is 100% bad for a character, 100% of the time. It could be those particular authors—Book of Exalted Deeds is a particularly poor book overall, not just Vow of Poverty—but it might suggest some serious deficiencies in Wizards’ understanding of the game.
Nonetheless, the notion of “coverage”—which I haven’t seen in any Wizards of the Coast publication—is very common and very important. The necessity of magic—which, again, for mundane classes means magic items—is very much baked into the game, even if it sometimes it seems like Wizards of the Coast themselves weren’t aware they were doing it. It’s why wealth by level becomes so utterly crucial, and why substandard wealth hurts the weakest classes most.