Mirran Crusader needs a big mention in the Melira POD matchup, as that's the main reason many are mainboarding it now. It tends to just end the game very quickly against Melira POD.
Good sideboard cards against Scapeshift:
Leyline of Sanctity
Runed Halo (naming Valakut, the molten pinnacle)
Fulminator Mage
Liliana of the Veil
Burrenton Forge-Tender (Protection against red sweepers)
Aven Mindcensor
Tectonic Edge
Good sideboard cards against GR Tron:
Stony Silence
Fulminator Mage
Aven Mindcensor
Tectonic Edge
Ghost Quarter
Burrenton Forge-Tender (Protection against red sweepers)
Pithing Needle or Oblivion Ring against Karn.
Good sideboard cards against Storm:
Rest in Peace
Leyline of Sanctity
Runed Halo (naming Grapeshot)
Liliana of the Veil (reducing their hand is important)
Zealous Persecution (killing goblin swarm)
Also Rule of Law, Ethersworn Canonist etc., but I don't think we need that narrow cards when we have so many other good answers.
Bogles:
Runed Halo (usually naming bogle or the hexporoof elf)
Liliana of the Veil (or other cards with an edict effect)
Mirran Crusader
Zealous Persecution (on the play you can sweep their 1/1 hexproof creature in response to an aura spell)
Enchantment removal like Disenchant or Oblivion Ring
Nahiri + Emrakul together with SFM + equipment will be too much clunk I think
Mono-White Martyr by Elias Klocker - Grand Prix Stockholm 2018, top 8
https://www.mtggoldfish.com/deck/1331217#paper
Jeskai Tempo by Shota Takao - Grand Prix Hong Kong 2018, 3rd Place
https://www.mtggoldfish.com/deck/1331260#paper
Ok, wasn't aware of that..
Dylan Donegan played GDS to 12-3 and 33rd place, so that's something.
At the very least it's faring better than the forgotten Ux Delver decks...
I gotta ask, does modern really need cards like Cavern of Souls, that invalidates counterspells completely? Cavern of Souls decks (Humans, Eldrazi Tron, GR Eldrazi etc.) are at the top of the table now (see SCG Milwaukee) and why the heck would you fill your deck with counterspells facing that...? And the saddest thing is that Wizards told us this is exactly what they want magic to be because they (don't laugh) think magic is unfun when creatures are countered... :
https://magic.wizards.com/en/articles/archive/latest-developments/gonna-hate-2012-04-20
Is Jitte even a maindeckable card? What decks would maindeck that? It's a pretty specific card in that it's good in certain creature decks mostly against weenie decks. Also, just like the swords (that sees no play in modern whatsoever), it has the inherent clunkiness that the creature you are trying to equip can just be bolted/pathed/pushed etc. in response after paying mana to equip. Or the equipment itself can be destroyed with Kol.Command, Decay, Abrade, Oblivion Stone, Detention Sphere, Reclamation Sage, Qasali Pridemage etc etc. If Jitte proves to be "Uber powerful" in modern (which I really doubt...), a slight uptick in these cards would probably balance that just fine. There's no shortage of ways to deal with artifacts in modern today.
And if Jitte would just be a good sideboard card in certain decks for certain weenie mathups, keeping it banned makes as much sense as banning Grim Lavamancer or Izzet Staticaster...
Snapcasters are so broken that 4 is reccomended if you have them, but I'm sure you're already aware of that.