These are all the reactions I can think of from the PHB which should be included in BG3:
Opportunity Attacks — get attack vs foe who leaves your reach
Ready Action — user-defined trigger and action
Bardic Inspiration — for at least saving throws - Bard 1
Beguiling Defenses — bounce charm back at initiator — Warlock 10 (Archfey)
Bend Luck — spend 2 sorcery pts to add/subtract 1d4 from someone's attack/save/check — Sorcerer 6 (Wild Magic)
Combat Inspiration — +1d6..1d12 to AC vs one attack — Bard 3 (Valor)
Counterspell — counter a spell being cast — Sorcerer/Warlock/Wizard 5
Cutting Words — -1d6..1d12 to opponent's attack roll, damage roll, or ability check — Bard 3 (Lore)
Dampen Elements — grant resistance vs one energy attack — Cleric 6 (Nature)
Defensive Duelist — add prof bonus to AC vs one attack if wielding finesse weapon (Dex 13)
Deflect Missles — deflect/catch-and-return one missle — Monk 3
Entropic Ward — impose disadvantage on one attack vs you, then gain advantage vs attacker — Warlock 6 (Great Old One)
Feather Fall — safe landing for <=5 creatures for 1m — Bard/Sorcerer/Wizard 1
Giant Killer — get attack vs large or larger creature who attacks you — Ranger 3 (Hunter)
Hellish Rebuke — 1d10+1d10/slot lvl fire to attacker — Tiefling 3, Warlock 1
Illusory Self — negate an attack — Wizard 10 (Illusionist)
Instinctive Charm — divert attack to another target — Wizard 6 (Enchanter)
Mage Slayer — get melee weapon attack vs adjacent foe who casts a spell
Misty Escape — teleport 60' when damaged — Warlock 6 (Archfey)
Opportunist — get attack vs foe who hit adjacent ally — Monk 17 (Shadow)
Parry (Maneuver) — reduce damage of one melee attack — Fighter 3 (Battle Master), Martial Adept feat
Polearm Master — get Opportunity Attacks when someone enters your reach with Glaive/Halberd/Quarterstaff
Projected Ward — negate some damage within 30' — Wizard 6 (Abjurer)
Protection (Fighting Style) — use shield to impose disadvantage on one attack vs adjacent ally — Fighter 1, Paladin 2
Retaliation — get attack vs adjacent foe who harms you — Barbarian 14 (Berserker)
Riposte (Maneuver) — get attack vs adjacent foe who attacks you and misses — Fighter 3 (Battle Master), Martial Adept feat
Sentinel — get attack vs adjacent foe who attacks someone else
Shield — +5 AC until next turn, castable *after* hit! — Sorcerer/Wizard 1
Shield Master — take no damage instead of half damage on successful Dex save
Slow Fall — reduce falling damage — Monk 4
Soul of Vengeance — get attack vs Enmity target when it makes an attack — Paladin 15 (Vengeance)
Spell Thief — steal a spell that affects you — Rogue 17 (Arcane Trickster)
Stand Against the Tide — redirect melee attack that misses you to another creature — Ranger 15 (Hunter)
Uncanny Dodge — halve damage from one attack — Rogue 5, Ranger 15 (Hunter)
Warding Flare/Improved Flare — impose disadvantage on one attack — Cleric 1/6 (Light)
War God's Blessing (Channel Divinity) — grant +10 on attack roll — Cleric 2 (War)
Wrath of the Storm — 2d8 lightning/thunder vs adjacent foe who hits you — Cleric 1 (Tempest)
Relentless Avenger — move up to half speed when you hit someone using an Opportunity Attacks — Paladin 7 (Vengeance)
Sentinel — drop foe's speed to 0 when you hit them with an Opportunity Attacks; prevent foe from negating your Opportunity Attacks trigger via the Disengage action
War Caster — cast single-target spell instead of melee attack for Opportunity Attacks
Commander's Strike (Maneuver) — allow ally to use their reaction to make one weapon attack — Fighter 3 (Battle Master)
Maneuvering Attack (Maneuver) — allow ally to use their reaction to move half speed without Opportunity Attacks from target — Fighter 3 (Battle Master)
Familiars: When you cast a touch spell while your familiar is within 100', it can use its reaction to deliver it.
I am playing in several pen and paper D&D 5e games each week and would miss having these available. Yes, some we'll not see due to whatever the level cap is but many of them are still missing from EA with the level cap of 4.
