It's if anything pretty standard to not include mixed races in a setting. People love to create made up racial hierarchies with no room for that, for reasons that I'll try not to read too much into.
Pretty much all Baldur's Gate's imitators have no mixed race option. Pillars of Eternity, Divinity: Original Sin, probably others I'm not thinking of, none to be found.
That being said, you're right that the optics are probably different when having a setting with mixed race characters, and then removing them. Plus, even without mundane stuff like half elves, it's pretty baked into the life's blood of DnD at this point that you can lay with demons and dragons and all sorts of nonsense and produce viable offspring. They'd get a lot of backlash if they were like "No more tieflings, dragon disciples, etc etc" but the optics would be even weirder if mixing was exclusively impossible when talking about the "lineages" that have occasionally been used as a metaphor for racial issues in their materials, even if they're now attempting to divorce themselves from that.