I think the shiny will be released with Psystrike during some event/community day like we saw with the legendary birds. They have enough time to slowly roll out legendaries with new moves and shiny variants to keep people raiding and for several Pokémon to powercreep above Pokémon that power creeped them before.
For example, Groudon. Groudon will likely be surpassed as a ground-type attacker in gen 4 by Garchomp, possibly Rhyperior, and possibly Mamoswine. After that happens, Groudon’s shiny form will probably be released alongside Precipice Blades.
The same goes for Kyogre with Origin Pulse, Rayquaza with Dragon Ascent, etc.