Sideboard Theory:

> R/B Midrange:

Their Problem Cards:
Sheoldred, the Apocalypse - Kills a non-indestructible Soulflayer if we have to block.
Graveyard Trespasser - Exiles important cards.
Bloodtithe Harvester - Can -4/-4 Soulflayer off of two blood tokens if no hexproof.
Go Blank - Exiles graveyards, importantly sorcery speed.
Extinction Event - Kills any Soulflayer setup.

In:
4x Abrupt Decay - Hits everything besides Sheoldred.

Out:
4x Thoughtseize - Forces the game into topdeck situation while also being a terrible topdeck.

> Nykthos Ramp:

Their Problem Cards:
Cavalier of Thorns - 5/6 with reach, will block and kill a non-Zetalpa Soulflayer.
Karn, the Great Creator - Access to sideboard and silver bullets game 1.
Storm the Festival - Generates a lot of advantage that is difficult to come back from.
Nykthos, Shrine to Nyx - Nykthos is manageable as long as we can keep devotion off the board and answer whatever they play.

In:
2x Pithing Needle - Almost certainly naming Karn. They need exactly Boseiju to remove it and it should slow them down enough that they can't fetch The Stone Brain.
2x Samut, Voice of Dissent - Double strike and haste help close out the game before they have time to stabilise.

Out:
4x Striped Riverwinder - They have practically no interaction aside from Karn, so hexproof is less important.

> Gruul Boat

Their Problem Cards:
Skysovereign, Consul Flagship - Very evasive to all of our removal as it'll most likely only be a creature on their turn. Needs a Zetalpa Soulflayer to get through it.
The Akroan War - Will steal a non-hexproof Soulflayer for multiple turns.

In:

4x Abrupt Decay - Destroys every creature in the deck.
2x Assassin's Trophy - At the cost of a basic land, kills any threat, the panic button.
2x Ritual of Soot - The recovery option, buys us more time to set up a Soulflayer if they've stabilised quickly or we're struggling.

Out:
4x Thoughtseize - Discarding one three drop only to have them draw another isn't great, don't need hand interaction in this matchup.
4x Spell Pierce - Everything that can be Pierced can also be hit with Trophy, but with better timing.

> G/W Angels

Their Problem Cards:
All the angels have flying, so our usual evasion won't be as good.
Deafening Silence - This isn't as bad as it looks as we can play a lot at instant speed, removed fairly easily by Abrupt Decay if it gets too taxing.
Shapers' Sanctuary - To slow them down enough to set up Soulflayer, we're going to need to remove their creatures, Sanctuary gaining them a card each time we do is pretty bad.

In:
4x Abrupt Decay - Removes every non-land permanent in their deck, always a good card to have in hand.
2x Ritual of Soot - Helps a Soulflayer get through fliers, lets us catch up if we're behind.

Out:
4x Striped Riverwinder - They have no removal that hits a 4BB creature, so hexproof isn't needed in the Soulflayer pile.
2x Thoughtseize - Discarding a Collected Company is nice, but they're a pretty resilient deck, cut two.

> Lotus Combo

Their Problem Cards:
Emergent Ultimatum - Tutors for their combo
Languish - Kills any Soulflayer setup.

In:
2x Ashiok, Dream Render - Blocks their ultimatums while not needing to be protected as much due to lack of creatures.
2x Samut, Voice of Dissent - Doublestrike and haste help close out the game before they have time to stabilise.

Out:
1x Fauna Shaman - A little slower than I'd like in this matchup.
3x Striped Riverwinder - Next to no targeted removal makes hexproof less important.

> U/W Control

Their Problem Cards:
Farewell - Exiles any Soulflayer setup.
Supreme Verdict - Kills a non-indestructible Soulflayer
Teferi, Hero of Dominaria - Can bounce a non-hexproof Soulflayer
Rest in Peace - Shuts down our entire gameplan

In:
2x Pithing Needle - Teferi can get out of hand quickly otherwise.
1x Haywire Mite - Rest in Peace is a big issue if they land it. Can be searched with Fauna Shaman
2x Assassin's Trophy - Hits both Planeswalkers and RIP.

Out:
2x Fatal Push - Hits next to nothing in the deck.
1x Otherworldly Gaze - Trimmed for more specific matchup cards.
1x Tainted Indulgence - Trimmed for more specific matchup cards.
1x Spell Pierce - With Lotus Field they often have enough mana to play around it, trimmed one, mostly to make space.

>Mono-White Humans

Their Problem Cards:
Brutal Cathar - Exiles a non-hexproof Soulflayer. Even if we get it back, it loses the keywords.
Brave the Elements - Allows them to get through damage, blank removal and force auras to fall off.
Rest in Peace - Shuts down our entire gameplan.
Thalia, Guardian of Thraben - as a low curve deck with few lands, adding 50%+ to the cost of our spells hurts.

In:
4x Abrupt Decay

Out:
1x Spell Pierce - Mostly creature based matchup, but still has some key non-creatures.
1x Otherworldly Gaze - Trimmed for more specific matchup cards.
2x Thoughtseize - Hand interaction is less important in the aggro matchup.

>Abzan Greasefang

Their Problem Cards
Greasefang, Okiba Boss - Reanimates and crews vehicles.
Parhelion II - Cheated into play
Eldritch Evolution - Finds Greasefang.
Liliana of the Veil - Can force us to sacrifice a Soulflayer, bypassing all our protection.

In:
4x Abrupt Decay - Kills Greasefang and Liliana unconditionally.
2x Ashiok, Dream Render - Mills us, exiles their whole graveyard, stops them tutoring with Eldritch Evolution.
1x Pithing Needle - Crew is an activated ability, so naming Parhelion II forces then to find removal.

Out:
2x Fatal Push - Doesn't kill Greasefang and all other creatures are only used for their etb effects.
1x Nightveil Predator - Deathtouch doesn't improve any blocks, hexproof doesn't stop any removal.
4x Striped Riverwinder - No removal hits a 4BB creature, so hexproof isn't needed.