If they don't want to use it anymore, why would the game still recognize it as valid input? Are you implying that it accepting the outmoded input as valid is intentional? Because it shouldn't do that. The game accepting invalid code as valid is a bug,
Perhaps I should rename the bug to "Superflat customization still recognizes and accepts classic ID inputs."
In which case this is still a bug; You see the screenshot I provided? I got that by using the code [[2;7,59x1,3x3,2,6:3;1;stronghold,biome_1,village,decoration,dungeon,lake,mineshaft,lava_lake}}. This scenario should never happen with the input provided. If the code is being depreciated then the game shouldn't recognize the code as valid.
Yeah, you're right. They should just leave it in there, no point in bringing unintended operation to their attention.