Features as of Minecraft version 1.19:

The overworld is unchanged compared to previous versions. Everything is working fine.
The nether won't generate many stuff if I flatten it so the best I could do was to remove the bedrock ceiling thus you can easily dig up to the flat surface of the world this way traversing the nether and getting flat surface for builds is still a viable thing.
The end is untouched.
Two editions: Regular, that only flattens the overworld and Plus that removes the bedrock ceiling in the nether too

Features as of Minecraft version 1.18:

Redone the data pack so now achieving flatness with different methods
Now aquatic biomes spawn too!
Jungle and desert pyramids are added back!
Everything seem to be working in order in the overworld
Mountainous biomes are obviously flat so there isn't too much to them
In plus version the nether and the end are both quite flattened, at least the top of them
In achieving that I accidentally made the ender dragon easier to beat since there are no obsidian monoliths and crystals... Sorry?
The nether is relatively safe to walk around but don't go diving deep in lava while using the plus version. For some reason where there is lava lake there isn't any bedrock. You might just... you know. Fall out of the world?

The world surface generates in only one plane, highly resembling to Mojang's original superflat mode, except for it is not but tries to achieve it by sticking to a given height
The world is still very ideal for both survival and creative modes for the majority of the world space is still empty as seen in the very first picture in the description
With the exception of oceans, ocean monuments and desert pyramids everything works
Ruined portals and stronghold still generate so you can "complete" the game if you wished so
Two editions. One that makes the overworld superflat, the other makes the nether and the end superflat too In the plus edition end biomes are a bit modified so chorus trees generate everywhere, giving the landscape a more dense look