The Shiny rate is unchanged (1/4096). However, the Shiny Charm can be obtained after catching all 721 Pokemon, not just the Kalos Dex.
: All Mega Stones (except Mewtwonite) and most TMs are available before the Elite Four. Updated lists are in the Item Changes & Locations document. pokemon eternal x guide verified
: All 721 Pokémon (up to Gen VI) are obtainable within the game. Rarer starters and "more desirable" Pokémon have lower encounter rates in the wild. New Evolution Methods The Shiny rate is unchanged (1/4096)