Before 21w06a, the world limit was 256 blocks. Now, it is 384 blocks. However, the superflat preset selector does not allow for creation of a world with over 256 layers, as the previous limits have not been changed.
is duplicated by
MC-214867Superflat worlds customization does not support hights above 256
Resolved
MC-214907Flat world generator's maximum is 256 instead of 384
Resolved
relates to
MC-213583Crash upon importing world generation settings with over 256 layers
Resolved
MC-214866Superflat worlds customization does not support negative hights
Resolved
MC-214951Superflat worlds start generating at negative coordinates, causing a dark horizon