What? How would MAP_32BIT reimplement an entire ABI? It has more in common with x86-64 than i386.
There aren't many syscalls that return pointers; mmap() is one of them, and using MAP_32BIT means that the pointer will be 32-bit.
32-bit code tells the kernel to allocate in the lower 32-bit address space (via MAP_32BIT) precisely so that the 64-bit pointers the kernel works with can be safely truncated to 32-bit user space pointers.
Notably, this means the actual pointers to that memory are still 64 bit.
Obviously, when you change pointers to be 32 bit, that requires an ABI change because the binary layout of your data structures is different.
You could do everything else in userspace if you really wanted. At the cost of safety, type handling and optimization from the compiler as all it would see is 64bit pointers being typecast and generic 32bit numeric data. Whatever you were looking to gain would be lost in extra work, and you can forget about debugging. But theoretically...