Suuuuuuuper late to the party, but hopefully this is still useful to folks.
First, one misconception I want to clear up: having a mode available only in customs doesn't reduce the work needed to make it available. The main work involved in bringing a mode back is making sure it still works with everything we've put in the game since its last run, including everything from champs, VGUs, functional changes to abilities, items, runes, skins, underlying code changes like how damage is recorded or how textures are loaded into game or how game results are recorded in match history, etc. Even if we were willing to let rotating game modes be crazy buggy for the sake of only turning them on for customs (we aren't btw), some bugs are bad enough that they can crash entire game servers, meaning hundreds of matches would crash—not just customs—so even then we wouldn't be able to avoid that preventative QA work if we want to bring a mode back for custom lobbies.
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