The RISC-V instruction set was carefully designed not to require the use of any currently valid patents to do an implementation. It is up to each processor designer to not violate any patents in their project.
https://riscv.org/technical/specifications/risc-v-genealogy/
Of course, the various extensions might violate current patents (I would guess that packed SIMD and cryptography extensions are particularly at risk). But the basic ISA does not use anything that was not already widely adopted by 2003.
Besides that, engineers in large companies are told to explicitly not look up any patents so they won't be accused of willful infringement.