I've a lot of decks running star of extinction in the sideboard, but I haven't really figured out what deck that's for. I guess it can clear a board full of PWs, destroy a flipped utility land and a board full of creatures, but the 7cc means it's only relevant late game and you'll have a hard time getting it through a counterspells deck.

Oh yeah forgot graveyard hate vs. Drakes. Edited. Unmoored Ego sounds really cute though, you can't even get card advantage with that card. It's not like the deck crumbles if you neutralize one of their drakes either, since they have two (+ Arclight Phoenix). I can get behind Ixalan's Binding more, since UR doesn't have ways to get rid of enchantments (I think).

Anyone have any idea what cards are good vs. Golgari (other than Tocatli Honor Guard)?

I got T3 unmoored ego playing Jeskai drakes, but he went looking for teferi... Oops. But then he T4 unmoored ego again and got my Arclights (this was best-of-1 ladder btw) and he still lost.

If he's running multiple Unmoored Ego main deck, i'm skeptical the rest of his build is good. And yes, Phoenix is the best Ego target in the format. Drakes is basically just a combo deck.

Well he lost against an Arclight Phoenix deck with no Arclights in the deck, so we can safely assume the deck isn't fantastic.
The only reasons I can see to unmoored ego main deck is in a lich's mastery deck so you don't auto-lose to Cleansing Nova or a mill deck against gaea's blessing (but nobody plays that anyway).

Yes, that's why this thread is about step 2: what cards constitute direct hate against each meta deck's core method of victory?

Has anyone tried Treasure Map vs. control? It looks reasonable along with Karn - it's not just that they are colorless, but also that Treasure map provides mana advantage if it flips (which allows you to double-spell through a counter) as well as extra cards. It also makes 3 treasures to make Karnstructs big, plus both Karn & Treasure Map are colorless -> fits in every deck.

Step 0: Build the best 60 card deck you can - this is usually not including hate cards, but rather just striving for maximum efficiency of your victory plan

Step 1: Figure out which decks beat you
Step 2: Find direct hate to each deck's core method of victory (or cards that will do better against opponent hate cards)
Step 3: Add all of those cards to a pseudo side board (This could be 30 cards at that point depending on what you are worried about)
Step 4: Whittle down your list based on your beliefs of likely frequency of encounters
Step 5: Do step 4 a second or third time if necessary

Step 6: Make a sideboard plan in advance for each of your expected opponents - or opponent categories - it makes no sense to include SB cards, if you don't know or can't pinpoint what the card is intended to replace in the original 60.
(Do not try to SB on the fly, you can make minor adjustments, but you should know your plan, against for example RDW, before the match, and ideally before you call your SB done).

Step 7: Be prepared to mulligan aggressively for your hate cards against certain opponents.

Or at least, this is what I do, when I build a deck.

Note: if you are highly favored against a deck, it probably isn't worth devoting a whole lot of slots to beating their most lucky draw. Just plan on beating them in 3 games, instead of wasting SB slots on beating them consistently in 2. (You may still have SB cards that get switched in, but they should be there to beat other decks you'll have a hard time against, and also just happen to improve a winning match.)