They've done similar events in the past, the one which immediately comes to mind is the "Pokemon Scraps" event. These event don't come to the US sadly.
Those scrap events were pretty much the only way to get many Mythical Pokemon with the Alola symbol. Even with the, however, there were still some Mythical Pokemon that could no be obtained in Gen VII, and Unown also was skipped for some reason(since they can't breed, Unown was banned from VGC by technicality, unlike Mythicals which are intentionally banned from it)