GROHE
GROHE Blue is built around a "separate inner waterway" that isolates the filtered drinking water from the unfiltered brass mixer path, which is a real design effort toward a clean drinking channel. The catch is that GROHE does not publicly state what that separate channel is made of, so whether the filtered water avoids plastic is exactly the thing you can't confirm. The isolation architecture is documented; the material of the part that actually matters is not.
How clear are their specs?
GROHE documents the separate-waterway architecture but won't name the material of that channel - the single spec that would tell you whether the drinking water touches plastic.
