Well, GCC 15 already ended support for the nios2 soft-core. The successor to it is Nios V which runs RISC-V. If users want us update the kernel, they'll also need to update their FPGA.

Microblaze also is a soft-core, based on RISC-V, presumably it could support actual RISC-V if anyone cared.

All others haven't received new hardware within the last 10 years, everybody using these will already be running an LTS kernel on there.

It looks like there really are no reasons not to require rust for new versions of the kernel from now on then!