This matter has been addressed several times and has been marked as resolved, working as intended. I disagree and want to open a thread to make this known.

It does not work as intended, because when you select a world generation to contain only deep oceans, it is very weird that you have to walk for days through uninhabited lands draped with deep sea structures to find something that slightly represents the wrong type of ocean.

Instead of just a redirection to that resolved bug report, I would like to have a clear statement that single biomes are no longer supported, which would be a reason for me to stop playing. Also, at least drop the option at the new world generation page. If not, please explain why that option is maintained as it clearly not working and there is no intention to get it working again.