Anyone want to discuss this and what cards are effective against which decks?

Monored
- anything that dies to Goblin Chainwhirler
+ answers to Experimental Frenzy
+ answers to Rekindling Phoenix
+ lifegain if you have it

As far as I can tell, monored stays aggressive post-board but they seem to have Treasure Map in their board quite often. They're also likely to have Banefire as an uncounterable finisher. Several authors have warned that when sideboarding against monored, you need to beat their sideboard, not their maindeck.

Control
- removal
+ planeswalkers
+ uncounterable or hexproof threats if you have them
+ The Immortal Sun if you have it
+ flexible removal is good, if they can also answer planeswalkers

Jeskai can potentially have Niv Mizzet, drakes, or Lyra (rare) post-board. Grixis and Dimir might have Doom Whisperer and Thief of Sanity.

White aggro
- expensive cards
+ sweepers
+ exile removal (to answer Adanto Vanguard)
+ answers to Experimental Frenzy

They can potentially have Aurelia post-board.

Golgari
+ card advantage
+ Tocatli Honor Guard
+ answers to Wildgrowth Walker if aggro

This is the matchup I'm least sure about general sideboard principles. Their deck is just a massive grind-based deck, so is the general idea to outgrind them? They can have Duresses and such to beat control decks after board, sweepers Golden Demise & Ritual of Soot, and Doom Whisperer + an array of planeswalkers as well.

Monoblue
- expensive removal
+ cheap removal

Kill all their stuff and they end up with lots of dead cards.

Selesnya
+ board sweepers vs. tokens
+ Duress vs. tokens
+ if they're keeping in removal, then answers to enchantments are good since their only removal is Conclave Tribunal & Ixalan's Binding

The midrange decks will have Lyra post-board if they didn't already have it. Otherwise they usually board into planeswalkers. There's also a weird plan of Thorn Lieutenant or Adanto Vanguard + Squire's Devotion vs. aggro decks. I've even seen a deck that planned to board in Adanto Vanguard against monored so they can put Squire's Devotion on it (monored cannot kill Adanto Vanguard) and lifelink the hell out of the matchup.

Drakes
+ Duress
+ answers to flyers, preferably exile-based to permanently answer Arclight Phoenix
+ graveyard hate

Don't know if cheap removal is worth keeping in. Cards like Baffling End and Lightning Strike can answer Goblin Electromancer, but might also do nothing.

Vs. Drakes, you want to board in things that exile creatures.

This includes:

Unmoored Ego
Deathgorge Scavenger
Blood Operative
Seal Away
Settle the Wreckage

I would rather give specific suggestions for a certain deck, how to build it's sideboard, but of course above suggestions do make sense. It's just very dependent on the deck you play and play against.

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.

Ixalan's Binding is hilarious vs Drakes.

Thats mostly to combat midrange decks like Golgari, where it also kills Carnies (and indeed with a bunch of PW's as bonus kills), not vs counter based stuff.

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)?

You'd be pulling Arclight Phoenix with Unmoored Ego, not the drakes. Without the Phoenix, you can 1-for-1 their drakes.

I was joking about Binding. It's terrible vs the phoenix, which is all that matters. Drakes die to cast down or lava coil, who cares about them.

Unmoored Ego puts you down a card though, and Crackling Drake replaces itself. I get that Izzet drakes runs relatively few win conditions so answering four of them is a big deal, but this still seems quite cute to me. Has anyone run this to success?

Also: has anyone run Treasure Map sufficiently to tell if it's effective as a plan against control? It seems reasonable - along with Karn, it's colorless, making it a possible option for all decks.

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).

Even in Lich's Mastery, it'd still be a sideboard card that you could go get with Mastermind's Acquisition. But even then, Lich's Mastery isn't beating Jeskai Control game 1 like ever lol. I've just accepted that fact.

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.)

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.

Well that was part of the reason I wrote what I did. I think it's best to think about these things in a slightly different way.

Which cards are the weakness, which replacements provide strength?

Or put another way, if you ask me what about Treasure Map? I'd want to know which card is getting cut, and why. Some decks might benefit more from going faster against control, rather than looking for a slow grind alternative against decks which want to play the slow game. Furthermore, are you even going to be facing control in sufficient quantity to put a card like Treasure Map in your deck or Karn in the first place?

Just some thoughts - not a critique of that SB plan per se.

Nice DJ, nice. You have a way with words my friend! SD

This was the sideboard I was going to run in my G/W deck:

4 x Nullhide Ferox
2 x Carnage Tyrant
2 x Vivien Reid
2 x Treasure Map
1 x Karn, Scion of Urza
2 x Knight of Autumn
2 x Baffling End

The first 11 cards are all meant as anti-control cards. What else can I run to make it even better vs. control?

TBH I've come down a lot on Ferox; especially in decks that don't run 30+ creature spells (Selesniya Tokes is just that). I'd cur 2 for more Carnage Tyrants.