As mentioned by Donat during his SummerCamp talk, s-maxage will be interpreted by (non reverse) proxies, and will keep cache alive for the default time with no PURGE possibilities, since those are out of our reach (enterprise proxies, mobile phones...):

• update our varnish recommended config to remove this header once processed  done in EZP-28126
• check if this could be done with the internal Sf RP  done in EZP-28126
• insist about it in the doc