I'm developing a campaign setting with my S.O. which we intend to publish. (We're uncertain of exactly where we'll publish yet.) I was reading through the D&D 5e SRD released under OGL to see what was and wasn't allowed for use. I noticed that Dwarves and Elves only have one subrace each, but that Duergar and Drow appear in the monster listing. Are they usable as subraces as well, then? Would I need to create my own interpretation of them as a player subrace?
You may not reprint the official Drow and Duergar subrace options
The monsters and the subraces are different mechanically, and those non-OGL subrace mechanics are exactly what you’re hoping to use. Although conceptually and in-game they’re related ideas, as far as publishing is concerned, subraces and monsters are unrelated bits of content.
The subraces are simply not licensed as open content in the SRD and may not be published in OGL-compliant products.
You could make your own
Your product could provide your own version of Drow and Duergar player subraces. They could not resemble the official ones though, as that would be obviously copying and modifying the original, something neither copyright nor the OGL allow. This would be very difficult, since the Drow and Duergar as we know them in D&D are original inventions owned by WotC, and even the names aren’t available in an SRD-derived product. You’d have to make up completely original Underground Elf and Underground Dwarf concepts and abilities.
Since your versions would have to be original and incompatible, there are only a few cases where it would even be worth it. A campaign setting is one reasonable purpose for original creations like that.
You don’t have to reinvent the wheel
However, there’s no reason to even include character creation in your campaign setting, so you could just avoid the whole issue entirely and direct people to the PHB and other books to make characters.
For your campaign setting–product purposes, you’d probably be better off simply listing “drow” and “duergar” with all the other races that exist in your setting and telling your readers to use the WotC books to make characters as normal.