At some point before the Caves and Cliffs updated world build limits I had created a flat world with creative builds. After C&C, the flat world was updated so that the player could build below the default bedrock layer (y:00). Activated experimental features and it worked fine. I built things below 0. No issues throughout the minor updates. 

Now that the Wild update has released, everything build below the bedrock layer (still at y:00) has been deleted save for a single brewing stand. Not sure why only that stayed while other blocks and entities of various types were deleted. Solids, semisolids, command blocks were all deleted. Only things below bedrock (ie. negative y coordinates) were affected, excluding the brewing stand I mentioned before.  

It's not the end of the world; it's just a bit discouraging.