Well I just googled it and maybe you're right, partially. Looks like Showdown was added to the rotation settings of some modes, I'm thinking it was available before if you specifically chose it in a private match but it wouldn't appear in the game if you were playing whatever map was randomly chosen. I think it's always been there though, and can't really find any evidence proving or disproving it aside from that little blip about one of the updates.
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