Splinterlands (aka Steem Monsters) has 21 unique battle rulesets. However, as far as I know, there's no list of rulesets. Until now. Here they are:
Standard
No modification to the standard gameplay rules and mechanics.
Lost Legendaries
Legendary Monsters may not be used in battles.
Back to Basics
Monsters lose all abilities.
Melee Mayhem
Monsters with Melee attack can attack from any position.
Taking Sides
Neutral Monsters may not be used in battles.
Silenced Summoners
Summoners do not give any stat buffs or debuffs.
Rise of the Commons
Only Common and Rare Monsters may be used in battles.
Aim True
Melee and Ranged attacks always hit their target.
Super Sneak
All Monsters with Melee attack have the Sneak ability.
Weak Magic
Magic attacks hit Armor before reducing Health.
Unprotected
Monsters do not have any armor and do not get armor from Abilities or Summoner Buffs.
Target Practice
All Monsters with Ranged or Magic attack have the Snipe ability.
Up Close & Personal
Only Monsters with Melee attack may be used in battles.
Reverse Speed
Monsters with the lowest Speed attack first and have the highest chance of evading attacks.
Broken Arrows
Monsters with Ranged attack may not be used in battles.
Fog of War
Monsters lose the Sneak and Snipe abilities.
Little League
Only Monsters & Summoners that cost 4 Mana or less may be used in battles.
Earthquake
Non-flying Monsters take 2 Melee damage at the end of each round.
Healed Out
All healing abilities are removed from Monsters.
Armored Up
All Monsters have 2 Armor in addition to their normal Armor stat.
Keep Your Distance
Monsters with Melee attack may not be used in battles.