Updated:

1. Make a void world

When creating the world, choose Superflat, open Customize, then Presets, and pick The Void. Start in Creative with cheats on.

2. Build the island

Fly to where the island corner should be and run:

/fill ~ ~-3 ~ ~5 ~-1 ~2 dirt
/fill ~ ~-3 ~3 ~2 ~-1 ~5 dirt
/fill ~ ~-1 ~ ~5 ~-1 ~2 grass_block
/fill ~ ~-1 ~3 ~2 ~-1 ~5 grass_block

This makes the classic L-shaped island with grass on top. See fill command.

3. Grow a tree

/place feature minecraft:oak ~4 ~ ~1

If it fails, pick a grass block with open space above. See place command.

4. Starter chest

/setblock ~1 ~ ~4 chest[facing=north]{Items:[{Slot:0b,id:"minecraft:lava_bucket",count:1},{Slot:1b,id:"minecraft:ice",count:2},{Slot:2b,id:"minecraft:melon_slice",count:1},{Slot:3b,id:"minecraft:pumpkin_seeds",count:1},{Slot:4b,id:"minecraft:bone_meal",count:3}]}

Lava and two ice blocks are the classic start for a cobblestone generator and an infinite water source. In 1.20.4 and older write Count:1b.

5. Fix the spawn

/setworldspawn ~1 ~ ~1
/gamerule respawn_radius 0

Run this on the island. The second rule is spawnRadius in 1.21.10 and older. Then switch to survival.

6. Teleport back after a fall (repeating)

/execute as @a at @s positioned ~ -100 ~ if entity @s[distance=..30] run tp @s 1 66 1

Players between y -130 and -70 are sent back before void damage hits. Change 1 66 1 to a spot on your island, or skip this for a harder game.

Troubleshooting

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