Once they release a shiny is it always in the game?

According to this article, yes:

Once a Pokemon is made available as a shiny Pokemon, it is possible to receive a shiny variant in any way the Pokemon is obtainable. Evolved Pokemon cannot be caught shiny. They must be caught shiny in the basic stage, then evolved. The exception to this rule is Pikachu (and now Nidorina and Nidoqueen), who can be caught shiny.