The mod “default” is basically just a mod like any other. There is nothing special about it. It just happens to use the name “default”. “default” is not a magic name, it carries no special meaning in Minetest. To answer your question, “default” is—as a mod—not default at all. In Minetest Game this is just the mod which adds a lot of basic stuff: Nodes, craftitems, crafts, fuels, furnaces, liquids, player model, sound templates, and so on. It may or may not be required to depend on default. See it as some kind of a “core” mod on which many other mods will depend on. So the name “default” is kind of a misnomer. The only thing what matters for dependencies is whether you are using anything the default mod provides. If not, you don't need to depend on default.
Yes, subgames have been forked from Minetest Game so there are multiple versions of default. It is of course possible to depend on any of those, but only one at a time! You have to be sure on which subgame's default mod you want to depend.
If you list default as a dependency when publishing the mod in the forums you should also specify the name of the subgame, especially when it is
not Minetest Game. Normally this info is omitted in which case the users will assume it is default from Minetest Game and
only from Minetest Game.
Then could people make a default default mod or something?
This question doesn't make sense since there are is no mod which can be considered as the mod which is used as the
default mod (as in: fallback, standard, etc.), there are only mods which have been
named “default”.