I believe m68k already has a working Rust compiler of sorts, though it's not part of the default Rust chain. I think shaping that fork into something that will let it run and compile like normal is feasible.

For other architectures currently unsupported by Rust, I doubt it'll happen. The CPU architectures themselves are long dead and often only used for industrial applications, so the probability of hobbyists getting their hands on them is pretty slim.

People still using these old architectures for anything but enthusiast hacking will probably not be using Debian Trixie, and if they do, they can probably find a workaround. It's not like the .deb format itself is changing, so old versions of apt and dpkg will keep working for quite a while.