Centaurs have the Fey trait (Guildmaster's Guide to Ravnica, pg. 16):

Fey. Your creature type is fey, rather than humanoid.

What does this mean in terms of gameplay? I know that certain spells, such as hold person, require a humanoid target, so a centaur wouldn't be affected by that spell. But what about any implicit mechanics?

In the intro to the Monster Manual (pg. 6) and the corresponding section of the basic rules, it says under "Type":

The game includes the following monster types, which have no rules of their own.

So does this mean that the only consequence of them having the Fey trait is that they cannot be affected by spells and effects that target humanoids, and can be affected by spells and traits that target fey, and that's it?

For context, the reason I'm asking is that I want to give the playable elven subrace Eladrin the Fey trait as well (to make them consistent with the "monster" Eladrin stats, where they are listed as fey), and want to know the mechanical consequences of doing so.

Many small consequences, some with class features, others with spells.

Mechanically speaking, yes, the only consequence of them having the Fey trait is that they cannot be affected by spells and effects that target humanoids, and can be affected by spells and traits that target fey. Humanoid-targeting features are a lot to list (some that come to mind are hold/charm/dominate person, lycanthropy, the warlock's Gaze of Two Minds eldritch invocation, calm emotions, crown of madness, or simulacrum), but below is a list of all the fey-related features.

Circle of the Land druids can't be charmed by Centaurs.

Nature’s Ward

When you reach 10th level, you can’t be charmed or frightened by elementals or fey, and you are immune to poison and disease.

Oath of the Ancients paladins can turn the faithless:

Turn the Faithless. You can use your Channel Divinity to utter ancient words that are painful for fey and fiends to hear. As an action, you present your holy symbol, and each fey or fiend within 30 feet of you that can hear you must make a Wisdom saving throw. On a failed save, the creature is turned for 1 minute or until it takes damage.

They can be a Ranger's Favored Enemy (without having to pick two specific races of humanoid) and are revealed by their Primeval Awareness:

Favored Enemy

Choose a type of favored enemy: aberrations, beasts, celestials, constructs, dragons, elementals, fey, fiends, giants, monstrosities, oozes, plants, or undead.

Primeval Awareness

For 1 minute per level of the spell slot you expend, you can sense whether the following types of creatures are present within 1 mile of you (or within up to 6 miles if you are in your favored terrain): aberrations, celestials, dragons, elementals, fey, fiends, and undead.

Arcana Domain clerics' Arcane Abjuration Channel Divinity option can affect fey:

As an action, you present your holy symbol, and one celestial, elemental, fey, or fiend of your choice that is within 30 feet of you must make a Wisdom saving throw, provided that the creature can see or hear you. If the creature fails its saving throw, it is turned for 1 minute or until it takes any damage.

You might be detected by commune with nature:

You instantly gain knowledge of up to three facts of your choice about [...] powerful celestials, fey, fiends, elementals, or undead

You are detected by detect evil and good:

For the duration, you know if there is an aberration, celestial, elemental, fey, fiend, or undead within 30 feet of you, as well as where the creature is located.

You are resisted by someone affected with dispel evil and good or protection from evil and good:

For the duration, celestials, elementals, fey, fiends, and undead have disadvantage on attack rolls against you

Forbiddance might harm you:

In addition, the spell damages types of creatures that you choose when you cast it. Choose one or more of the following: celestials, elementals, fey, fiends, and undead. When a chosen creature enters the spell's area for the first time on a turn or starts its turn there, the creature takes 5d10 radiant or necrotic damage (your choice when you cast this spell).

Hallow stops you from entering, unless the caster makes an exception:

First, celestials, elementals, fey, fiends, and undead can't enter the area, nor can such creatures charm, frighten, or possess creatures within it. Any creature charmed, frightened, or possessed by such a creature is no longer charmed, frightened, or possessed upon entering the area. You can exclude one or more of those types of creatures from this effect.

Magic circle can affect you:

Choose one or more of the following types of creatures: celestials, elementals, fey, fiends, or undead. The circle affects a creature of the chosen type [...]

You can be bound to serve someone with planar binding:

With this spell, you attempt to bind a celestial, an elemental, a fey, or a fiend to your service.

You can be left out of a temple of the gods:

The temple opposes types of creatures you choose when you cast this spell. Choose one or more of the following: celestials, elementals, fey, fiends, or undead. If a creature of the chosen type attempts to enter the temple, that creature must make a Charisma saving throw. On a failed save, it can’t enter the temple for 24 hours. Even if the creature can enter the temple, the magic there hinders it; whenever it makes an attack roll, an ability check, or a saving throw inside the temple, it must roll a d4 and subtract the number rolled from the d20 roll.

Some spells force a fey back to its homeplane, but I don't think Centaurs have a different home plane, so ignore those. With a home-brewy DM, you might be able to conjure a centaur with conjure fey, or conjure woodland beings.

You are immune to spells and class features that target humanoids, but vulnerable to those that can target fey.

Class features that can target fey creatures are the Ranger's Favored Enemy and Primeval Awareness, the Oath of the Ancients Paladin's Turn the Faithless, among others.

You are immune to spells like Hold Person, that target humanoids specifically, but vulnerable to many others like Protection from Evil and Good, Magic Circle and Hallow.

By searching on the books, I gathered the following lists:

Spells/Class Features that fey are immune/can't be targeted

• Animate Dead


• Calm Emotions


• Ceremony


• Charm Person


• Crown of Madness


• Dominate Person


• Finger of Death* (immune to rise as a zombie only)


• Gaze of Two Minds (Warlock)


• Hold Person


• Magic Jar


• Mantle of Whispers (Bard of the College of the Whispers)


• Reincarnate


• Simulacrum


• Words of Terror (Bard of the College of the Whispers)

Spells/Class Features that affect fey, but don't affect humanoids

• Arcane Abjuration (Cleric of the Arcana Domain)


• Commune with Nature


• Conjure Fey


• Conjure Woodland Beings


• Detect Evil and Good


• Dispel Evil and Good


• Divine Sense (Paladin)


• Divine Word


• Favored Enemy* (Ranger, can affect humanoids)


• Forbiddance


• Hallow


• Magic Circle


• Nature's Ward (Druid of the Circle of the Land)


• Planar Binding


• Primeval Awareness (Ranger)


• Temple of the Gods


• Turn the Faithless (Paladin of the Oath of the Ancients)

You are also immune to lycanthropy

Fey creatures cannot be werewolves or weretigers (or any other kind of lycanthrope) as its curse effects only affect humanoids. The "Player Characters as Lycanthropes" sidebar located at page 207 of the Monster Manual says (emphasis mine):

A non-lycanthrope humanoid hit by an attack that carries the curse of lycanthropy must succeed on a Constitution saving throw (DC 8 + the lycanthrope's proficiency bonus + the lycanthrope's Constitution modifier) or be cursed.