Source: Matty Love Gaming's video
In fact, according to Riot's FAQ on the matter at the moment of posting this:
Showcases are approximately 3–4 week long promotions within a single theme and feature new champion skins and accessories to match. They are also accompanied by a number of Showcase missions and rewards.
Which, in fact, still implies that we should have missions with the Broken Covenant Skin Showcase, and here is a screenshot
Showcase Missions are missions, not unlike Event Missions, that accompany a new Showcase debut. Missions can be viewed in the Panel below your friends list in the client.
Now it says:
Before we go into the details, just a quick note that not all Showcases have missions! If that's the case, your only objective is to grace the Rift with your stylish new look.
Showcase Missions are missions, not unlike Event Missions, that accompany some Showcases. Missions can be viewed in the Panel below your friends list in the client.
Not only were they sneaky about it, which is hilarious considering their recent promise of communicating better, but as you guys can see in the very first question, its still implied that all Showcases come with missions and rewards, so even their sneaky attempt at covering that up failed hard.
UPDATE: Riot Rovient responded to this situation.
External link →
Apologies, players. This one is on me! I was unaware that we had a commitment to Showcase missions. We'll make sure we get these free rewards into an upcoming Event in addition to existing content.
Missions will also return in future Showcases.