The other example of nested functions which you've not mentioned was in Metaware High C.

There they allowed nested functions, but also what they termed "full function values", being a form of fat pointer. Certainly I came across it in High-C v1.7 in 1990, and the full manual for an earlier version (1.2?) from around '85 can be found on Bitsavers.

It had a syntax like:

    extern void Quick_sort(
      int Lo, int Hi, int Compare(int a, int b)!,
      void Swap(int a,int b)!
    );

    static Sort_private_table() {
      Entry Entries[100];
      int Compare(int a,int b) {
        return Entries[a] < Entries[b];
      }
      void Swap(int a,int b) {
        Entry Temp = Entries[a];
        Entries[a] = Entries[b];
        Entries[b] = Temp;
      }
      ...
      Quick_sort(1,100,Compare,Swap);
    }

The above is an extract from their language reference, which you can find here:

https://archive.org/download/Yoshizuki_UnRenamed_Files__D-V/...