Everything is legal.
The ban list for CEDH and other competitive Commander formats.https://mtgcommander.net/index.php/banned-list/
Cards that deny players from drawing cards repeatedly without giving an alternative; like "Narset of the Vail" and "Notion Thief", but cards like "Shared Fate" are allowed.
Cards that allow you to search your library for non-land cards.
Cards that do not allow playing, activating, triggering abilities and/or removing, replacing abilities for more than a single card per player. Cards like "Pithing Needle" are allowed, but not cards like "Ether storm".
Cards that allow you to make a player go net negative with on field land count. Cards that destroy a land but allow the controller of the destroyed land to put one into play from their deck are allowed.
Cards that allow you to take an extra turn, force a player to skip their turn and or allow you to control a plyer during their turn.
Cards that prevent permanents from untapping indefinitely, example "Imi statue", but cards like "Impede Momentum" are allowed.
Cards like "Fury" that allow you to play themselves using an alternative casting cost without paying mana. Cards like "Frogmite" are allowed because you are paying mana for it's regular casting cost, even though the mana cost has been reduced to zero.
Permanent cards like "Sol Ring","Nadu, Winged Wisdom" and/or “Dockside Extortionist” that are capable of generating more mana than they take to cast on the same turn you play them.
Any card on wizards reserve list. WOC Official Reprint Policy
Cards that unconditionally add conditional mana cost onto player actions like drawing, attacking or casting spells. Example = "Rhystic Study". However cards like "Kappa Cannoneer" are allowed because the player can choose a different target to avoid being given the option to pay the cost.
This is kinda hard to define. If you discover an infinite combo during game play don't use an iteration of the infinite/semi-infinite loop more than twice per turn. If you do, you forfeit before resolution. A non-exhaustive list can be found here: https://commanderspellbook.com/
Cards like "Moat" that indefinitely prevent battle from your opponent's creatures to you.
Eminence, Commander Ninjutsu, and other abilities that can be activated or applied from the command zone.
Cards like "Wrath of God" or "Cyclonic Rift"
Cards that give players poison counters.
Cards that repeatedly gain the user life like "Life Chisel"
Cards the repeatedly remove permanents from the battlefield like "Hullbreaker Horror"
Non permanent cards that generate more mana than they take to cast.
Cards that give you more cards in hand than when you played them like "Archmage's Charm".
Cards that let you filter the cards you would draw like "Preordain".
Cards that put lands into play or produce mana.
Creatures with effect text.
Lands that do not have the "basic" type.
Instant speed spells, fash and/or abilities that can be activated at instant speed.