I finally encountered a turn-one combo deck; it works like this:

1. Mulligan to find Mutavault and activate it.
2. Discover Transmogrify.
3. Transmogrify your Mutavault into Cultivator Colossus. It’ll die instantly.
4. The rest of your deck is all lands, and most importantly, features all of the OTJ crime deserts. Draw them all, put them all into play, deal lethal damage and then some.

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I played this deck for a couple of hours this evening, and my recordkeeping may not be exact, but I’ve gone something like 57 - 9.

At the last minute I cut the Cabaretti Courtyards and four Forests to make room for playsets of Stormkeld Vanguard and Sword Coast Serpent. I don’t know if it was worth it or not – I rarely cast either, but I also never had trouble getting the mana I needed, so it’s probably fine either way.

Losses

• Opponent went second and discovered Vanishing Verse, exiling my first enchantment. If it had been in the graveyard there would have been some hope, but there’s no getting it back from exile.
• Opponent went first and discovered Teferi, Hero of Dominaria. Tucked my first enchantment on their turn 2. No problem, I’ll just rediscover it, right? I did, and they cast March of Otherworldly Light on it. (A few games later another opponent also discovered Teferi on their first turn, but I had gone first, so I just untapped and won.)
• I drew Sanguine Bond on turn 2 (and didn’t feel like I had a reasonable chance of surviving to turn 5).
• Opponent’s first discovery was Archive Trap, and it milled one of my enchantments. I didn’t have a Skyturtle, but stuck it out to see if I could draw one. Once they’d milled all four, I had no outs.
• Opponent went first and discovered Gilded Lotus, then Goblin Charbelcher. I actually drew Exquisite Blood on turn 2, but it didn’t even matter; they had the win and were just slow-rolling it. (I did beat some Charbelcher decks – the match-up mostly depends on who goes first.)
• Lost to another turn-2 combo deck: Quintorius Kand plus Spark Double.
• Drew Sanguine Bond. Survived long enough to cast it, but lost to Nicol Bolas, Dragon-God’s ultimate right afterwards.
• Another Charbelcher deck, and I drew Sanguine Bond on turn 2. Wrecked their first Charbelcher but they just got a second.
• And yet another Charbelcher deck. I’ve been lucky to go first against them more often than not.

Other notes

• The deck is very reliable. Accidentally drawing one of your own enchantments so you can’t discover it is the biggest risk.
• The Colossal Skyturtle backup plan has worked a couple of times. The most interesting was one where my opponent went first and discovered Weave the Nightmare. They didn’t heist either of my enchantments, but they did get a Stormkeld Vanguard and used it to destroy my first enchantment. I chose patience, and worked on thinning my library and gaining life to stay alive. By a stroke of luck, I drew a Skyturtle just in time. I was able to re-cast my enchantment and play a Thornwood Falls with lethal damage on the other side of the table.
• The second-most exciting win was against an opponent who went first and discovered Railway Brawler, then Gigantosaurus to make a 20/20.
• Very few people were playing landless decks, so I didn’t explore the Drannith Magistrate prison idea I talked about previously. Most of the landless strategies that I did see were Charbelcher decks. Magistrate might be good against them, but no better than the Life Lock plan I was already playing – both match-ups go to whomever wins the die roll.
• I’d estimate that a third or more of players just grabbed a Historic deck and didn’t specifically build for the format (nothing wrong with that, I’ve done it for plenty of Midweek Magic events myself).
• Apologies to anyone who lost to this ridiculous nonsense. At least it was quick?

Additional thoughts:

• If we’re losing our enchantments too often, we could build in some redundancy by going up to two copies of each. The problem with that is there’s a 33% chance you’ll get a duplicate on turn 2, in which case you’d have to wait until turn 3 or 4 to win. So I guess it’s worth it if our enchantments are getting killed in more than 33% of games.

• I was wrong about Teferi in particular, the Historic-legal version is the “fixed” Alchemy card and that doesn’t get in Chimil’s way. Other can’t-cast effects, such as Drannith Magistrate, are still a concern.

• Looking at the list of weaknesses I laid out starts me thinking about how I’d build a white prison deck for this format. Unfortunately, it would need a lot of rares and I don’t have quite that many wildcards.