I don't know whether anyone has ever really threatened a patent lawsuit over the ISA, but it definitely does seem to be a good idea to document the origins of instructions, and that they are either so well known and obvious that they can't be patented, or else that they were patented and the patent has expired.

Another protection is that the membership application for RISC-V International includes certification that RISC-V does not infringe on the applicant's own patents. With companies such as Intel, AMD, IBM, Marvell, Microchip, MIPS, nVIDIA, NXP, Qualcomm, Renesas, Samsung, Siemens, Sony, STM, Texas Instruments being members that covers a lot of ground.

The base ISA (up to RV32GC, RV64GC) very deliberately breaks no new ground. It's just smoother, with known potholes filled in.

Many of the optional extensions do break some new ground. The V extension has some unique features, though the basics are inspired by the Cray-1. RVWMO is based on experience gained with ARM and DEC Alpha memory ordering models, but fixing the problems -- and was actually designed by a world-wide group of academic and industry experts. Both of these are examples of something that simply would not be done as well by a small group of people inside a single company, working to a deadline.