RISC-V was originally a teaching ISA but because of it's licence and open design process it's increasingly being adopted by industry.

There are other open ISAs available but they are generally bigger targets (POWER is open now but it's a result of decades of IBM-work so it's not as initially simple as RISC-V).

Also one thing that even on HN seems to be missed - RISC-V IS THE ISA not the silicon. All it means is that you don't have to pay the designer of the ISA any royalties to use it a la Arm, the CPU itself is most likely still a black box.