On undisclosed dates
It’s not ideal, but a lot of those missing Mega Stones from Pokemon Sun and Moon are coming at some point. And take note of that key phrasing “at some point,” and through undisclosed means by way of distribution events. Yuck!
Mega Stones have been a contentious part of Sun and Moon‘s pro scene, as all of the stones aren’t actually present in the game, leaving many to the mercy of these upcoming events to actually obtain them. It’s not really a new thing for Game Freak though, as some Pokemon are even fully data-mined (with complete models and movesets!) for up to a year and change before they actually get a distribution of some sort.
At least it’s not paid DLC I guess? Except for when some GameStops try to scam unsuspecting customers and claim “you have to pre-order the game” to get their distribution codes.