Will the skin comeback as a challenge or as a purchasable item or will it just disappear for ever
The Demon Hunter Sombra skin is unlikely to return, this is because it was originally a BlizzCon 2018 Virtual Ticket Skin and considered a bonus collection item rather than a Halloween one. In any case there is no specific details from Blizzard at this time. Be sure to follow updates from the Overwatch Twitter Feed to make sure to not miss out on any cool skin opportunities like this one.
Please note, this is not the best place to ask about collectibles in Overwatch, if you need help posting in the general discussion forums please check this thread: