Creating a superflat world causes the experimental world warning to show, even though superflat words are not an experimental world generation feature. Now, this might be a consequence of the reforming of the entire world generation system, but I doubt it is intended.

To reproduce:

1: Create a new superflat world.
2: You will see the experimental world settings message, even though superflat worlds are not experimental

UPDATE: Apparently this is a known issue that applies to all worlds. Please close this issue