Did not see this coming, people who test things out in Ultimate have done fine without it, but what we see here is actually a fine piece of art. Who knows what other familiar modes might come through in later updates, Homerun Contest for instance. I myself even did a Petition recently after learning about Stage Builder coming to Ultimate about Smash Run: https://www.change.org/p/nintendo-bring-back-smash-run-in-a-future-update-in-ultimate

Good luck I say with how we get amazed by people’s efforts with the stages that they create, certain familiar stages that didn’t make it into Ultimate though might actually be quite hard to recreate in a way to look like how they were originally due to how they were handled: https://www.ssbwiki.com/Stage_transformation