Why not play Angler ? You need another good TBR target besides shadow and it is a solid threat on its own.
Also, with all the discard spells won't Tidehollow often be just a 2/2 bear with nothing to take ?
It is also a pretty bad draw later in the game so I don't think that it is better than Dreadhorde Arcanist.
Especially with Unearth I'd prefer to run Remorseful Cleric and just exile the yard instead of shutting down some abilities.
As a flying threat it can also clock them before you need to pull the trigger on theit GY.
A problem I had with the deck is being run over smaller creature decks so I consider putting back something like Pyroclasm in the SB to deal with them.
This is a very unsual looking list, I guess it is heavily geared towards beating the "Hogaak Bridgewine" decks with all the GY hate
(Surgical + Spellbomb) it is packing.
The 4x SB Yixlid Jailer is bizzare imho, how is it better than Remorseful Cleric (or Leyline/Trap/Ashiok) ?
I never considered Dreadhorde Arcanist for the deck but without cantrips I don't know how useful it is.
It reminds me a bit of Abbot of Keral Keep which is also good with a low spell curve.
Giver of Runes on the other hand seems like a great idea, to protect the shadows and help push through blockers.
When building the deck some time ago I considered Emerge Unscathed for a similar purpose but eventually discarded the idea.
I don't really see the point of Mishra's Bauble here, as this deck doesn't have delve cards to "feed" and isn't trying to turn on Delirium.
I'd probably just play 2x more Faithless Looting and maybe 2x Night's Whisper instead.
I've played some matches with the deck and so far I was rather impressed with Lightning Skelemental and Unearth but the sample size is still too small to draw any conclusions yet.
Lunge isn't as good if you are playing an attrition game rather than some kind of a combo-kill game (together with Battle Rage). Unearth is also never a dead card without a target in the GY (or against GY-hate) as it has cycling.
Unearth getting back 3-drops is the main thing it has going for it (otherwise Claim // Fame is better) and it makes Faithless Looting better as it is easier to pitch creatures knowing to can easily get them back.
Some more potentially playable 3-drops might be:
With the upcoming repring of Unearth I wonder if this deck can become more viable as it will be able to more effectively run out 3-cmc threats in addition to DS and Angler which it was really lacking compared to the Grixis/Jund versions which had far better 2-drops.
Being able to get back 3-drops is a good upgrade over Claim // Fame and being able to cycle it compensates over the loss of the back half.
One of these 3-cmc options can be Lightning Skelemental as it has good damage and hand disruption.
Also, with all the discard spells won't Tidehollow often be just a 2/2 bear with nothing to take ?
It is also a pretty bad draw later in the game so I don't think that it is better than Dreadhorde Arcanist.
A creature which I think can be good in the deck but nobody seems to play is Lightning Skelemental which works very well with Unearth and Faithless Looting (much better than Mishra's Bauble in the deck).
Chandra, Acolyte of Flame can make a decent Snapcaster impression in the deck.
https://www.mtggoldfish.com/articles/modern-mayhem-mardu-shadow-modern-magic-online
19x creatures seems a bit high though and I wonder why not play Anglers at all.
As a flying threat it can also clock them before you need to pull the trigger on theit GY.
A problem I had with the deck is being run over smaller creature decks so I consider putting back something like Pyroclasm in the SB to deal with them.
(Surgical + Spellbomb) it is packing.
The 4x SB Yixlid Jailer is bizzare imho, how is it better than Remorseful Cleric (or Leyline/Trap/Ashiok) ?
I never considered Dreadhorde Arcanist for the deck but without cantrips I don't know how useful it is.
It reminds me a bit of Abbot of Keral Keep which is also good with a low spell curve.
Giver of Runes on the other hand seems like a great idea, to protect the shadows and help push through blockers.
When building the deck some time ago I considered Emerge Unscathed for a similar purpose but eventually discarded the idea.
I don't really see the point of Mishra's Bauble here, as this deck doesn't have delve cards to "feed" and isn't trying to turn on Delirium.
I'd probably just play 2x more Faithless Looting and maybe 2x Night's Whisper instead.
I've played some matches with the deck and so far I was rather impressed with Lightning Skelemental and Unearth but the sample size is still too small to draw any conclusions yet.
4x Death's Shadow
4x Lightning Skelemental
4x Gurmag Angler
4x Street Wraith
Sorcery (16)
4x Thoughtseize
2x Inquisition of Kozilek
4x Faithless Looting
3x Unearth
3x Lingering Souls
Instant (9)
3x Lightning Bolt
3x Path to Exile
3x Fatal Push
4x Bloodstained Mire
3x Marsh Flats
3x Arid Mesa
2x Blood Crypt
2x Godless Shrine
1x Sacred Foundry
1x Silent Clearing
1x Mountain
1x Plains
1x Swamp
1x Fatal Push
1x Path to Exile
1x Gideon of the Trials
1x Rakdos Charm
1x Anguished Unmaking
2x Fulminator Mage
1x Painful Truths
3x Leyline of the Void
2x Nihil Spellbomb
2x Stony Silence
Another option could be to go with Ranger-Captain of Eos over Lightning Skelemental and maybe split it 3/1 with Goblin Rabblemaster.
Ashiok, Dream Render might be a better SB card than Leyline as it hates on search effects besides graveyards.
Ravenous Trap might also be worth consideration.
Unearth is also never a dead card without a target in the GY (or against GY-hate) as it has cycling.
Unearth getting back 3-drops is the main thing it has going for it (otherwise Claim // Fame is better) and it makes Faithless Looting better as it is easier to pitch creatures knowing to can easily get them back.
Some more potentially playable 3-drops might be:
# Monastery Mentor (might be a bit slow) which can be a consideration over Lingering Souls perhaps.
# Sin Prodder - can be a card draw engine here, especially good with Angler/Street Wraith as high-cmc cards which are not cast for the full cost anyway.
# Goblin Rabblemaster/Hanweir Garrison - can generate tokens to take over games.
# Ranger-Captain of Eos (gets DS) and Brimaz, King of Oreskos also seem pretty good though WW is hard for a BR based deck to cast.
Being able to get back 3-drops is a good upgrade over Claim // Fame and being able to cycle it compensates over the loss of the back half.
One of these 3-cmc options can be Lightning Skelemental as it has good damage and hand disruption.