Well, as far as they appear in my systems. ; )
I just recently came to a decision that I hadn’t even realized I was chewing on in the recesses of my mind, a decision on how to approach demi-humans in my systems. In short: my demi-humans are actually humans, just from a different origin world. So, the dwarves are humans from a distant homeland and some got transported to the prime setting in the distant past. The elves are humans from a distant homeland and some got transported to the prime setting in the distant past. I’m still wishy-washy about the existence of any other demi-humans — keep changing my mind about gnomes — and I’m not worrying about any others right now. The dwarves and elves, though, they’re simply “humans from other worlds” and that makes things a whole lot easier for my setting.
No need to worry about half-elves or half-dwarves, as I can say any offsprng would more closely resemble one line of parentage than the other and be largely identical to that line to where no mechanical differentiation is necessary. Your half-elf PC is thus either treated an an elf or as a setting-standard human and that’s all the consideration necessary. Players are free to fuss about parentage as they choose and the system doesn’t care; the PC will roll out as a dwarf or elf or human with characteristics accordingly.
This allows for the “demi-humans” to show some variance from the setting-standard humans without requiring any sort of complicated considerations for species. Dwarves are shorter, stockier humans with other characteristics based on evolving on a different world, with elves being longer-limbed and so forth due to their origins. I suspect it’ll also remove the “humans in funny suits” factor from the table, too (though putting the final descriptions into play with playtesters will shine light on that hypothesis).
It also works to make the appearance of other human strains easier to digest, so my half-formed thoughts of gray men may make it into usable form. It also has ramifications that extend to the place of humanoids in settings and like considerations. It helps to take advantage of my concept of the primary setting world bumping up against other planes regularly and thus being peppered with connections to lots of other places.