One of the primary reasons elves continued to lose ground against the relentless humans as they trudged across Cerilia in the early days of recorded history is the fact that the elves completely lacked divine magic. Elves are far more in tune with the magic of the land (mebhaigl, as the setting calls it, pronounced "MEE-vahl" because who cares about consonants), and make natural wizards, but they never had clerics as the humans did.
This is an important setting distinction, one that's pretty hard to handwave unless you modify the history a bit (or simply imply that the elves lost just because humans are fecund and relentless). Introducing old edition class restrictions seems like an unwelcome throwback, given how later editions of D&D are all about player choice and open selection of classes.
"Dwarf mages from 3rd edition onward? Preposterous!"
- Some guy, probably
My take is going to keep the idea of restricted class access based on race. It both keeps the important historical elements and also provides crucial flavor for the setting, but I introduce a possible solution below if someone just really, really wants to play a historically-improbable character.
Elves are restricted from belonging to the cleric, druid, and paladin classes. They have access to some minor magical healing via bards if they so choose, but divine magic is simply not the purview of the elves -- even the druid, which in other settings is iconic for elves. Druids are the priests of the nature god Erik, worshiped by humans.
Dwarves are restricted from belonging to the bard, eldritch knight fighter path, arcane trickster rogue path, sorcerer, wizard, and warlock classes. Cerilian dwarves are not magically endowed, but have access to divine magic as normal. Though nothing explicitly forbids them, you'll almost never see a dwarven Erik priest (also known as a druid).
Halflings are restricted from belonging to the eldritch knight fighter path, sorcerer, and wizard classes. I feel as though a strong enough case can be made for the iconic halfling bard, as well as the arcane trickster. Halfling warlocks seem unusual, but Great Old One path warlocks, reflavored for Shadow World entities, are very appropriate in my opinion (see below for more about warlocks). They wouldn't be Lovecraftian monstrosities, but eldritch creatures that still dwell in the Shadow World, or dead gods, might be suitable sources of power.
While my approach is heavy-handed, nothing stops you from allowing a very determined player to belong to one of these forbidden combinations. I can see a strong case being made for elf Oath of the Ancients paladins, for example. It should be very clear that the player is an abnormality -- perhaps the dwarf sorcerer's power is a strange manifestation of their bloodline abilities, or the elf druid believes more in living in harmony with humans and nature than remaining isolated and bitter towards everyone else.
It should remain clear that they are unique, however, and no NPCs of those race/class combination exist in significant numbers.
Modifications or Special Rules for Classes
There are a few classes that have a hard time fitting into Cerilia. For a purist interpretation of the setting, some changes need to be made.
The monk's theme and powers have a hard time translating into any of the cultures in the setting, and they're really iffy to reflavor into something like a battle friar or pugilist. You could potentially have a dead-set player who wants to be a monk come from a different part of the world (of which extremely little is known or catalogued). They would be a fish out of water, and having them also be blooded would border on ridiculously unlikely.
Sorcerer also has some problems, but mostly because of the setting's take on arcane magic and how it is made available. It's less egregious than the monk, however, and a case could be made for the draconic sorcerer who spent too much time (wittingly or unwittingly) near Cerilia's staggeringly powerful and ancient dragons. Like the possibility for the elf cleric or the dwarf wizard, you would be playing an exception to the rule and likely be alone in the world.
Some players and DMs are cool with that though! Far be it from me to say that it should Never Be Done.
Even more brow-furrowing than outright forbiddance is the restriction on wizards. Standard Birthright wholly prohibits unblooded/non-elven or half-elven individuals from accessing higher magic in the setting; mechanically, spells belonging to schools other than illusion and divination above second level. Given the restriction, it is extremely unlikely that players would choose to play a non-elven or half-elven wizard (known as a magician) unless they are also blooded, though sometimes you get those wacky players that love the self-imposed challenge. Magicians should otherwise be relegated to NPC only.
I think, and I'm drawing this from the PDF I keep linking, that it is reasonable to extend this to third level if someone really wants to be a magician. Of course, third level spells contain the Combat Trifecta -- that is, fireball, lightning bolt, and haste. The latest edition has good mechanics for making two of these stronger by using higher level spell slots, so even an unblooded magician has the potential for suitable destructiveness (though nasty stuff like cloudkill and meteor swarm remains forever out of their reach unless they somehow become blooded later on).
Of course, this restriction complicates the hybrid class paths for fighter and rogue (eldritch knight and arcane trickster, respectively), but they don't even get fourth level magic until levels 19 and 20 (and even then, only one spell). I feel they can break this restriction so as not to create a tracking problem for two measly levels that most campaigns never even reach.
The bard is kind of a gray area. They're arcane, but they also have a smattering of traditionally "cleric" spells. Do they have the same restriction? I'm leaning toward no -- an elven bard does slip in the possibility of magical healing that may fly in the face of their setting-traditional lack of healing magic, but in no sane universe is a 5th edition bard going to be the party's main combat healer in difficult combats (and if they are, that DM needs to amp up the challenge a little, in my humble opinion!).
The warlock remains untouched, and my justification for that is that the power of a warlock does not come from Cerilia's unique magic-of-the-land, but an external source such as a powerful fiend (demons and devils are extremely rare to see summoned, it seems), archfey (who would be acting as a conduit from the land themselves, or powerful unknowable entity from the Shadow World -- or even a particularly strong awnsheghlien (for the uninitiated, someone or something that has a mighty bloodline but is highly evil and corrupted).
Barring any revisions I may want to do based on comments and feedback I receive, I think this will be the only part I do on classes. Next we start into stuff that is completely rebuilt and not just converted, that being the addition of the Bloodline ability score as well as how players go about creating a blooded scion.