Minecraft 18w20c, Minecraft 18w21a, Minecraft 18w21b, Minecraft 18w22a, Minecraft 18w22b, Minecraft 18w22c, Minecraft 1.13-pre1, Minecraft 1.13-pre2, Minecraft 1.13-pre3, Minecraft 1.13-pre4, Minecraft 1.13-pre5, Minecraft 1.13-pre6
By generating a world using the Customized world setting before its removal, loading up the game in a version after this takes place, selecting the world and Re-Creating it, and then heading to More World Options, will show "Customized" as the world type.
The Customize button just makes a sound, and cycling away from Customized will not have it show up again. Generating the world seems to generate a default world.
If the post-removal-generated world is again recreated in a version where customized worlds still exist, it will generate customized, and remember the preset used.
This bug does not need to be fixed.