There's two stacked issues in this question.
Can a warforged smell things at all?
If they can, does being able to smell but not needing to breathe protect it from foul smells?
Starting with the second question, the game doesn't say warforged can't smell, and we generally assume that you can do normal human things that are not explicitly excluded.
However, it does say they don't need to breathe, and there is precedent for not needing to breathe granting protection from bad smells. The spell stinking cloud explicitly says creatures that don't need to breathe automatically save. It doesn't say creatures that can't smell are immune, only ones that don't breathe, which suggests that those things go together.
Similar language is not present in other situations involving bad smells, though; for example, the troglodyte's stench and the ghast's stench have no such clause, which means that non-breathing creatures must still save against the poison effect.
But that said, this is a weird sort of corner case, because Warforged represent the only non-breathing PC race, and the only (semi-)construct race. As a late addition to the game, earlier resources may not have even thought about the possibility of PCs that don't inhale gasses, so such oversights could reasonably be chalked up to "warforged are weird" and be house-ruled accordingly.
In his appearances on the Dragon+ YouTube videos, Jeremy Crawford has mentioned a couple of times that spells and abilities intentionally don't call out all the exceptions and corner cases, in the interest of making their basic effects clear and concise rather than bogging them down in an infinite tangle of minutia. The DM is expected to make rulings on those cases when they're necessary.
So, ultimately, there's no specific rule that says Warforged are immune to bad smells or can close off whatever chemical detectors they may or may not possess to simulate a sense of smell; but there's a precedent for a DM to rule that bad smells don't bother Warforged and/or that they don't even have a sense of smell, and the designers intend DMs to do that when it seems appropriate in their own game.
And of course, the Wayfinder's Guide lives somewhere in the limbo between playtest content and an official release. The devs have already stated that the 5e Eberron book coming out in November has updates to the Warforged, among many other things previously released in the Wayfinder's Guide, and may address questions like this more clearly -- but don't hold your breath. (geddit?)