SJS, that’s a new one for me. Why would a baby carrier be considered a breach of tznius? If tznius is about not drawing attention unnecessarily then I’d say that a carrier promotes tznius, since a baby in a carrier is often calmer & quieter than one in a stroller. Plus by using a carrier one can sometimes avoid using a double stroller, which is usually bulkier & more intrusive than a single stroller (or forgo a stroller altogether).
I used carriers for years & never thought there were any tznius issues involved.
(Of course I’m not trying to say that tznius requires that one uses a baby carrier, just that I don’t think they’re relevant to tznius at all).