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As mentioned here, customized worlds can't be opened in this snapshot.
More than 1 biome (rivers don't count) can generate in customized single-biome worlds
Customized World Preset Biome ID >= 8 adds 2
Max dirt height defaults to 256 in Customized world (max world height is 255)
Mesa (Bryce) doesn't use some settings from custom World
Page navigation buttons overlap page title on Customized world screen
The customization GUI initially doesn't have the 'Defaults' button enabled when recreating a world with customized settings
Bugged water generates above custom sea-level in swamp biomes
In Customized settings' page 1, 'Previous Page' button is unlocked after resetting defaults
Crash when generating world with certain custom preset
Invalid "Dungeon Count" when importing a "Customized" world preset
Ores with Spawn Size '1' or '2' don't appear in the Customized world
Missing spacing for scroll bar in customized world generation settings (fourth page)
Text fields in customized world generation do not enable repeated events
Resizing the game enables all GUI elements in customized world generation settings
Text fields do not deselect when clicking in another text field
"Defaults" button of customized world generation always disables when "Customize" is clicked
Select Structures spawning underground with customized settings.
depthNoiseScaleExponent doesn't do anything
Woodland Mansions will not generate when biome size is changed
Forest and taiga type biomes spawn with large treeless areas when ore vein spawn sizes are set to excessive values
Pressing 'Defaults' in customized world settings makes options unresponsive
"Drought" custom world preset has inaccurate icon
RIP Customised World Types?
customizable worlds are not supported with new aquatic snapshots 18w07a and 18w07b, message says something like thi world is unsupported and can only be played on 1.12.2 pls fix
customizable worlds are not supported with snapshots 18w07a and 18w07b
No Custom World Type
You can't get "Customized" world type
Customised worlds can't be created
Where is the "Custom World" in the New World menu?
Cannot play customized 1.12 world
Customized world type missing when creating new world
Customized world type is not appearing
No Customised option in World type
The custom surface of the world generator
"Customized" World Type not appearing
the custom world button is gone when i want to create a new world
Custom worlds have been removed
Unsupported world! /18w16a/
Custom map type not available when creating a new world
where are customized worlds?
You can't play customized worlds in 1.13
Custom world creation; current custom worlds cannot be opened