I think we're talking about three levels (or more, but for simplicity's sake) of "thematic maturity:"
One, vanilla D&D.
Two, low fantasy/sword & sorcery/grimdark. This would include Conan, GoT, post-apocalyptic themes, Dark Sun.
Three, gratuitous/explicitly dark/edgy themes. Book of Vile Darkness, Erotic Fantasy, etc.
The first is the baseline of the core product line. It is listed as 12+, so the implication is borderline PG-13 due to the emphasis on violence and perhaps various concepts that aren't for kids younger than middle school, with the caveat that many kids start playing earlier. But this, at least, gives us a sense of what WotC has in mind.
I would think that the second category would be implied at "14 or 15+" and the third "18+" but of course we haven't seen any such products in the current edition.
The question, then, is there anything intrinsically wrong or overly problematic with WotC having some products in the "15+" category? And if so, should they label it as such? Would that diminish sales enough to make it prohibitive for publication, even if the vast majority of players are 15 and older or simply ignore such guidelines?
I would think 18+ products are a niche market and for that reason alone--limited sales--WotC wouldn't bother with any of the resulting hassles. While I haven't scoured DM's Guild for such material, I'm sure it exists.
But I don't think it would be economically prohibitive to have some products labeled as 15+ or 14+. I mean, there's a huge fan-base for grimdark, and even stuff that isn't overtly grimdark can incorporate more mature, be it slavery or complex politics or environmental themes or simply sword & sorcery.
(I don't think sword & sorcery is inherently for more mature audiences only, but I can see why they'd label it as such. But the question is, should they create sword & sorcery products in the spirit of the tradition, or make "child-friendly" sword & sorcery that may lose something in translation?)