When creating a new world, under world types, there used to be a super customisable world type that basically gave you everything you ever wanted. This included Single Biome worlds amongst countless other features. There were several pages of settings.

But today in light of the new update Aquatica, I was trying to create a world that was all ocean cuz I couldn't find any deep oceans in the world I spawned initially, but I noticed that the option to create a customised world type was gone. Only superflat, normal, large biomes and AMPLIFIED.

I don't see any reason they would've removed this feature, so I'm guessing it's a bug.