It's not. It is an intrinsic characteristic of the Rioplatense dialect (as well as others), and completely devoid of sociolectal connotations. You might be referring to the aspirated S at the end of a word when it is not followed by a consonant (such as "¡vamo'!" instead of "¡vamos!", or "¿qué hacé'?" instead of "¿qué hacés?"), which is indeed associated with a lower class.
If you are a native speaker of the Rioplatense dialect, try to hear yourself in a normal conversation and you will notice that you won't say "esss verdad" but "eh verdad" (the "h" here being an aspirated S, not a Spanish silent H). Like in any other language, most native speakers don't stop to think about these things, they'll do it naturally. Try asking the average Spanish speaker on the street the Subjuntivo rules - they won't even know what Subjuntivo is and likely never noticed it existed, even though they use it daily.