What I expected to happen was...:
Creating a custom input under Superflat presets (i.e., 2;7,59x1,3x3,2,6:3;1;stronghold,biome_1,village,decoration,dungeon,lake,mineshaft,lava_lake,) to save and load a custom world exactly as specified.

What actually happened was...:
The game will immediately "forget" the custom preset upon clicking the "Done" button, and will revert to "Classic Flat" mode. (Note that the game still recognizes it as a valid input, see Attachment 1.)

Steps to reproduce:
1. From the main menu, select "Singleplayer", then "Create New World", then "More World Options...".
2. Select "Suerflat" from the "World Type:" button and press "Customize."
3: Select "Presets"
4: Type any valid input into the text box along the top of the screen. For example...
2;7,2x3,2,3:2;1;
2;7,59x1,3x3,2,6:3;1;stronghold,biome_1,village,decoration,dungeon,lake,mineshaft,lava_lake
2;7,2x3,33x2;1;village
5: Select "Use Preset", and then "Done."
6: Either select "Create New World" or "Customize" again, and observe how your changes were not saved.

This does NOT reproduce when using the new name system. (3;minecraft:bedrock,32*minecraft:dirt,minecraft:grass;1;village)

Speculation:
Either the classic IDs (2;7,2x3,2;1;village) are being depreciated in favor of the new names (3;minecraft:bedrock,2*minecraft:dirt,minecraft:grass;1;village,) in which case the old IDs should not be accepted as valid inputs, as seen in Attachment 1, or they are not being depreciated and they game won't save them anyway. In either case, something isn't operating as it should.