memory protection would be hard I think, right? The whole OS was based around message passing by just linking a message to the target process message port pointer.
edit: but, as per your other comment, you know that. How do you see protection working in amigaOS? Just share the pages that are needed for messaging and protect everything else?
A flag day would be needed. This means literally breaking the ABIs. New interfaces would take care of setting up shared memory or doing cross-space copying.
There's always ways to deal with old software, such as through virtualization.
that's why the later OS releases after amiga/commodore imploded extended some of the memory APIs to say "This needs to be globally visible". That way you'd allocate message memory in globally accessible memory and the MMU could do "things".
(Not that I'm a fan of that solution, just saying, they had to hack around it.)
edit: but, as per your other comment, you know that. How do you see protection working in amigaOS? Just share the pages that are needed for messaging and protect everything else?