Sandsifter is dealing with the problem of "how do I find valid instructions in any 1-15 byte sequence, which yields 2^120 combinations?"

In RISC-V there are currently only 2 byte and 4 byte instructions, which you can brute force your way through. The specification does technically allow for longer sequences, in which case sandsifter will work just fine. [And the RISC-v privileged specification has existed for years].