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.
Cannot reproduce in 18w30b. Must have been fixed at some point. Can you please confirm that this is fixed?