Hi ! I haven't found an answer elsewhere, so I try here.
I have seen many presets, and most of them explicitly mention each vanilla Map feature as "true".
Let's say I create a world with this kind of preset, then I later install a mod which adds a Map feature in a biome. Example : The Betweenlands add a structure in the swamp biome. Will it generate in new chunks ? Or is there a parameter to add in the preset to set a default true value for Map features ?
When you're speaking of mods it depends on how the mod maker approached adding that feature in the code. I think many coders would try to take the easiest route by just using processes already in the game and putting in their custom modifications into that to get the result they want. However some coders might be savvy enough that they make their own subroutines that are un-reliant upon a process already in the game.
So using your example of a swamp structure, take a look at how the game handles such things in the presets code. If the modder just added his swamp structure into the 'biome_1' category as a structure that spawns in swamps and allowed the game to generate based upon it's default processes then yes it would matter whether your preset code had 'biome_1' true or false. However if the modder just added his own subroutine to the world generation code that told it to add the structure to swamps then your preset code wouldn't effect that structure.