Are nested structured bindings possible?

No, it is not possible.

I distinctly remember reading somewhere that nested structured bindings are not allowed for C++17, but they are considering allowing it in a future standard. Can't find the source though.

No, they aren't possible; but this is:

for (auto&& [key, value] : data) {
  auto&& [my_int, my_float] = value;
}

which is close at least.