I should warn you that this topic in the past has provoked sufficient outrage from some quarters that it ended up getting closed. Some people are rather touchy about what a DM chooses to leave out of a campaign world. Hopefully we don't go that route this time.
Things not in my homebrew:
Gnomes: Technically, gnomes exist as a rarely encountered myth of the early world as a race of free peoples that were created by the gods after the godswar and then where abandoned and shortly after went extinct. Most scholars agree that such myths have no basis in fact or are distortions of some other truth, as the substance of the myth is fundamentally flawed and there is no actual evidence of such a people ever existing. The few scholars that think the myth has any basis, believes its a misinterpretation of some other well known fact. This myth I have some ideas about, but have never followed up with in my campaign world. However, gnomes/deep gnomes and so forth are certainly not an extant race nor available as a PC race. You could play a Forest Dwarf or a Sidhe with gnome like appearance and outlook and capture the experience quite well, but that's not quite the same as affirming that gnomes exist. As for why gnomes don't exist, I simply don't like them and don't feel they have any really broad and interesting space of their own. The standard gnome trope seems to imply that for the whole race there is but a single personality and culture, and that's in my opinion not broad enough for a PC race.
Halflings: I'm not a big fan of having this race outside of Tolkien's Middle Earth. And even in Tolkien's Middle Earth, Tolkien - to my knowledge - never explained the cosmological significance of the existence of Halflings. Their purpose in the story was clear enough, and their inspiration in the English rural peoples was equally clear, but they always seemed to be a bit of curiosity even in the setting designed for them. I feel 'halfling' is a bit broader of a trope than gnome, and has shown a bit more diversity in its handling by authors, but I'm still not won over by them. In game, you could play a human pygmy (although this would be a bit exotic, as they are about as rare in world as they are in real life) or a Sidhe with Halfling like features and capture much of the experience. Again, notice that the Sidhe as I understand it is inherently broad enough to encompass the whole of gnome and Halfling cultural tropes as presented in D&D, and still have more space left over for other character concepts than both races combined. This fits my desire for broad and diverse PC races.
Orc: This was the first thing I ever excluded formally from my homebrew about 25 years ago now. Following reading an article in Dragon that emphasized that the gods of the bugbears, hobgoblins, and goblins were all members of the same family of dieties, and indeed to a large extent shared between the races I decided that goblins were just inherently cooler than orcs and had a more interesting trope and internal interaction. They had at one end of the spectrum the coolness of Tucker's kobolds, and at the other end the brute force of '99 Bugbears' (Bard's Tale reference). It had been bothering me for some time that orcs and goblins were both 'ugly evil' races with little to differentiate them (both being inspired by ethnic groups of the same Tolkien race, that's hardly surprising), and so I just decided to drop 'orcs' entirely. Much like Halfling, Orc strikes me as something that made more sense solely within a Middle Earth world, whereas Goblin was an idea with a bit wider lore. Some writers have gone the opposite direction, and made orcs central and goblins a subspecies, and I suppose that's fine, but I had 'hobgoblin' for any of the orcish/Klingon honor tropes already that you see in say WoW now and stuck with it.
Demons/Devils: Functionally this isn't true, though technically it is. Originally this got started as a way to avoid provoking occult scare by adults. The longer I went without them, the less I found I need them and the less interesting the 'demons as just another sort of monster' idea that is trope D&D seemed. I noted for example that demons are all the time allowed to invade the world, but angels don't counter invade and help out their victims (and when authors have them do so, stupidly, invariably, they always act like demons anyway which has to be absolutely dumbest repeated trope in fantasy, and the only time it was ever played out in a way that made sense at all and was set up properly was Babylon-5). I also noted that it was really weird to have vaguely judeo-Christian inspired angels/demons, and yet strongly polytheistic inspired settings - Greek Gods and also Michael the Archangel and also Satan... ok, whatever. So gradually, I started replacing demons and devils with various culturally non-specific evil spirits with a more horror inspiration, subtler methods, and strongly different behavior than ordinary physical monsters. If I ever used any D&D stock demons or devils, they'd be unique beings that I'd borrowed lazily to represent a particular evil spirit - maybe using a Pit Fiend, but representing it not as a generic general of Hell, but as a unique attendant to a certain evil deity. In this context, demon or devil is just a non-specific layman term for any monstrous spiritual being - whether an animal spirit with an animosity to humans, a fire elemental of a particularly nasty disposition, a spirit that incarnated the emotion of horror, a blood-drinking ghost, or greater servitor of an evil deity. I've only very rarely used any of the normal D&D fiend stat blocks in my games, and most of the times when I have done so their origin wasn't recognized amongst the general random Cthulhuan weirdness you'd encounter in my games.
Drow: Functionally this is true, but technically it isn't. Everyone in my campaign world knows Drow are extinct. They were genocidally exterminated by the other elven racial groups during the Kinslaying some 5000 years ago. No Drow have ever been observed since. No PC is allowed to be Drow. Techically of course, I've got Drow in reserve should I ever feel the need to bring them into my game in an interesting way, and if I ever did I'd subvert a lot of the expectations. Also, of course, while Drow are presumably extinct, their spirits still exist in the underworld under the care of their still not dead mistress Lloth. For elves, the vengeful spirits of the Drow are one of the more feared 'demons', and many Drow still exist in an undead form - banshees, ghasts, etc. - even after all this time in darker corners of the world.
Probably at least 50% of all published monsters: I don't individually note 'this monster doesn't exist in my world' in most cases, but probably the majority of published monsters I'll never find a reason to use.
Probably at least 80% of all published spells: I don't usually note the spell doesn't exist, but if isn't in core, the vast majority of additional material for spells in my game is homebrewed.
