Artifact will not offer mod support - at least not at launch, Valve have confirmed. The Dota card game won’t have the Workshop features that its MOBA sibling does, according to today’s presentation of the upcoming card game.
Here's everything we know about Artifact so far. Expect this to be updated soon.
All of this is filed under the “not yet” column, however. Dota 2’s Workshop allows modders to create custom content for inclusion in the game, rated up by players before getting added to client. All those items have to be cosmetic in order to avoid disrupting the balance of a competitive game, and today's presentation by Game Newell suggests it would be difficult to allow opportunities for mods here “beyond illustrators.”