This is a server-side only mod (works with unmodded clients!) which allows beacons to grant creative flight to players when certain configurable conditions are met.

Note: Although this is a server-side mod, it can still be used in single player on clients

Behaviour

By default, the mod requires the beacon to be an activated level 4 beacon (i.e. a max level beacon with an effect active) and requires the player to have the "Sky's the Limit" advancement unlocked. If both these conditions are met, the player will be able to fly when within range of the beacon.
If the player goes out of range of the beacon, the flight will last the same amount of time as the potion effects. Once the flight expires, the player will be given the Slow Falling effect for 10s, allowing them to glide back down safely.

Configuration

This is all configurable, along with requiring the player to have certain items equipped or held. Here are all the config options:

minBeaconLevel specifies the minimum level the beacon must be to grant this effect
mainHandItem specifies the item, if any, which needs to be held in the player's main hand
offHandItem specifies the item, if any, which needs to be held in the player's off hand
anyHandItem specifies an item, if any, which needs to be held in one of the player's hands
headItem specifies the item, if any, which must be worn on the player's head
chestItem specifies the item, if any, which must be worn on the player's chest
legsItem specifies the item, if any, which must be worn on the player's legs
feetItem specifies the item, if any, which must be worn on the player's feet
advancementsRequired specifies the list of advancements that the player must have unlocked*
xpDrainRate specifies the amount of XP used per tick of flying (0.25 works well) (v1.1+)
flightLingerTime specifies how long the flight lasts when not in range**
slowFallingTime specifies how long the Slow Falling effect is given for after flight is lost
logLevel specifies the level of logging in the console, can be INFO, WARN, ERROR or OFF

* The id of the advancements is required. This can be found in the in-game autocomplete for the advancement command, or a full list with descriptions may be found on the Minecraft wiki.

** A flightLingerTime of 0 gives the same time as the effects for that beacon level. The time should also be more than 4 seconds if custom, as beacons only apply their effects every 4 seconds.

The following is an example config that requires a max level beacon, requires the player to hold feathers in both hands and have equipped full diamond armour and have the "Sky's the Limit" and "Return to Sender" advancements. It will grant flight for the same amount of time as the potion effects, will use 0.25 XP per tick of flight and gives 10 seconds of Slow Falling upon losing flight:

{
"minBeaconLevel": 4,
"mainHandItem": "minecraft:feather",
"offHandItem": "minecraft:feather",
"anyHandItem": null,
"headItem": "minecraft:diamond_helmet",
"chestItem": "minecraft:diamond_chestplate",
"legsItem": "minecraft:diamond_leggings",
"feetItem": "minecraft:diamond_boots",
"advancementsRequired": [
"minecraft:end/elytra",
"minecraft:nether/return_to_sender"
],
"xpDrainRate": 0.25,
"flightLingerTime": 0,
"slowFallingTime": 10,
"logLevel": "INFO"
}

The config file is present in a folder called "config" in the same directory as Minecraft, which is the server directory in case of a dedicated server, or .minecraft in case of the client (it will be in the same place as the mods folder). It is called beaconflight_config.json.

If the file doesn't exist, the mod will create a default file upon startup.

Credits

Thanks to @xAlicatt for the ideas, commissioning the mod, and for testing!

BeaconFlight