While in principle I think it would make a lot of sense for different human cultures (which in a traditional historic or fantasy world would heavily align with different human ethnicities) to have different plausibly culturally based strengths (such as a list of skills to choose a free proficiency from, knowing a cantrip, etc.), aside from a whole pile of racism issues most folk don't want to touch with a ten foot pole there's to me the even more damning factor which is that D&D is designed to be used with WotC's campaign worlds or with your own. If different cultures have a game impact then world building requires either accommodating the official ones in one's setting (possibly repurposed with new names, etc.) or creating new ones with all the balance and design implications of that. I think this is a major reason why when D&D does differentiate several subcultures or subspecies for their non-humans they are often fairly abstract things without a lot of clear meaning which could be imported into most worlds without trouble (Shield Dwarf vs. Mountain Dwarf, Lightfoot Halfling vs. Stout Halfling).
Elder Scrolls approach seems viable enough when designing species/culture/ethnicity/race for one specific setting. Firstly it is far less racially problematic for all Nords, for example, to have a resistance to cold damage and a few extra ranks of two-handed weapon proficiency in a setting where the worldbuilder can dictate that basically all Nords were raised within their culture and in the cold climes of Skyrim, than it is to make those propensities of a group in a more cosmopolitan setting where a substantial number of Nords would have been raised outside their culture and homeland. But secondly the races don't have to be particularly friendly to being easily repurposed to another, completely different setting. They just have to work for Tamriel.
The Cosemere RPG that came out last year, and which is basically a heavily modified 5e, leaned heavily into ethnic differentiation between humans, and that is a key aspect of the setting as I understand it (I played through the Starter Set but have read none of the books). I don't recal what exaxtly characters got from their ethnic background, but clearly the concept is still alive and well in D&D adjacent gaming for setting specific rules sets. But its more trouble than its worth for more setting agnostic games, and these tend to shy away from it.
As much as I am not a Daggerheart fan (it's fine, I just have to deal with too many people who are over the moon for a game I consider to be a lot of big swings with hit and miss results), I would suggest that its "communities" approach where players chose what type of community they originated from, while to my taste poorly executed in that game (no small rural agricultural community in a quasi medieval fantasy game? Seriously?), is probably the best sort of approach to capture cultural differences within a species in a setting agnostic way.
