@Tiger Guy and @Cameron Crane have covered several things. Two more subclass abilities:
- Beast Barbarian (3rd) Form of the Beast: Tail option lets you add 1d8 to your AC for one attack, as a reaction. It's quite good. If you pick this option when entering a rage, you can use it every round.
- Wild Magic sorcerer (6th): Bend Luck - When another creature you can see makes an attack roll, an ability check, or a saving throw, you can use your reaction and spend 2 sorcery points to roll 1d4 and apply the number rolled as a bonus or penalty (your choice) to the creature's roll. You can do so after the creature rolls but before any effects of the roll occur. Like Cutting Words, this is mechanically equivalent to an AC boost if used on an attack against you.
Similar, subtracting from attack rolls, but have to declare before the roll is made, not after you see the number:
- Druid Circle of Stars (6th): Cosmic Omen - Woe (50% chance at start of day to have this option available): Whenever a creature you can see within 30 feet of you is about to make an attack roll, a saving throw, or an ability check, you can use your reaction to roll a d6 and subtract the number rolled from the total.
A few other subclasses have reactions that impose disadvantage, or can cause an attack to miss separately from AC. Unlike Cutting Words, these aren't mechanically equivalent to boosting your AC for one attack. The disadvantage ones mostly require you to react before seeing the attack roll, unlike most of the Shield-like reactions, although some are like Silvery Barbs and can force another d20 roll after the fact.
Light Cleric (1st): Warding Flare - impose disadvantage as a reaction to an attack made against you.
Great Old One Warlock (6th): Entropic Ward - impose disadvantage before a creature rolls an attack against you, and grant yourself advantage on your next attack. (Like a limited form of the Silvery Barbs spell).
Hexblade Warlock (10th): Armor of Hexes - if hit by an attack from your hexblade's curse target, impose a 50% chance for the attack to miss regardless of the roll.
Protection fighting style (e.g. Fighter): impose disadvantage on an attack against someone else.
Rune Knight Fighter:
Cloud Rune (3rd) - when you or a creature you can see within 30 feet of you is hit by an attack roll, you can use your reaction to invoke the rune and choose a different creature within 30 feet of you, other than the attacker. The chosen creature becomes the target of the attack, using the same roll. This magic can transfer the attack's effects regardless of the attack's range. 1 use per short rest, but it automatically succeeds at making the attack not hit the target, and potentially hit an enemy.
Storm Rune (7th level): Until the state ends, when you or another creature you can see within 60 feet of you makes an attack roll, a saving throw, or an ability check, you can use your reaction to cause the roll to have advantage or disadvantage. Activating that prophetic state is 1/short rest.
Runic Shield (7th): When another creature you can see within 60 feet of you is hit by an attack roll, you can use your reaction to force the attacker to reroll the d20 and use the new roll. proficiency mod times / long rest.
Echo Knight fighter (10th) Shadow Martyr - can have one of your echoes teleport in to be the target instead, 1/short rest.
Drunken Master monk (6th): Redirect Attack - 1 Ki point to make an attack against you hit a different creature of your choice within 5 ft of you.
Gloom Stalker ranger (15th): Shadowy Dodge - impose disadvantage on one attack that didn't have advantage.
Mastermind rogue (13th): Misdirection - When you are targeted by an attack while a creature within 5 feet of you is granting you cover against that attack, you can use your reaction to have the attack target that creature instead of you.
Clockwork Soul sorcerer (1st): Restore Balance - force a straight roll instead of advantage or disadvantage on any d20 roll. proficiency times / long rest.
Chronurgy wizard (2nd): Chronal Shift - force a reroll of any attack/save/check for any creature within 30 ft, keeping the 2nd roll (so can help allies, unlike Silvery Barbs). 2/long rest.
Enchantment wizard (6th): Instinctive Charm - attacker has to succeed a Wis save or target the creature closest to it, other than you or it. Unlimited uses, except not on the same creature after they succeed the save once, until you take a long rest.
Illusion wizard (10th): Illusory Self - interpose an illusory duplicate, causing an attack to automatically miss. 1/short rest, and it's a reaction to a creature making an attack roll, so you don't get to see the result first.
Chronurgy wizard (14th): Convergent Future - When you or a creature you can see within 60 feet of you makes an attack roll, an ability check, or a saving throw, you can use your reaction to ignore the die roll and decide whether the number rolled is the minimum needed to succeed or one less than that number (your choice). Costs 1 level of exhaustion that can only be removed by a long rest. You'd usually want to use this to land high-level save-or-suck spells after burning through legendary resistances, or on persuasion or insight, but it can be used for making one attack automatically miss.
There are of course quite a few damage reduction / mitigation reactions, some by a fixed amount, others halving it or giving resistance, notably Uncanny Dodge (rogue 5th / Hunter ranger 15th option). If you search for "reaction" in each class listing, you'll find many. My browser doesn't have an option for case-sensitive search so searching for AC didn't work.
There are also many things that boost AC temporarily that aren't reactions, e.g. Swords bard Defensive Sword Flourish or Battlemaster Bait and Switch.
I looked through all the officially published subclasses (up to TCE) for all classes, but not races or feats or items.
