I assume you mean because the assembler was manually migrated in later Minix versions, not because there is a tool which can do so automatically. Or did I miss this?
> and a lot of interesting stuff was left out
Can you please make examples what you mean specifically?
One example is the new compiler driver that can use ACK and GCC from a single compiler driver and that can automatically convert assembler and figure out which archiver to use to create libraries.
Another example is library support for filenames longer than 14 characters that was completely transparent. MINIX3 just broken all backward compatibility by increasing the size of directory entries to a new fixed size.
I'm sure there is more stuff, these are just a few I remember.