As long as you are talking about RISC-V itself and not about a specific implementation, the RTL is neither open source nor not open source because there is no R

As long as you are talking about RISC-V itself and not about a specific implementation, the RTL is neither open source nor not open source because there is no RTL. RISC-V is an instruction set specification. As a CPU hobbyist, what this gives you is not a single step towards tapeout, but rather a working software-side toolchain (assembler, compiler, ...) right from the start, plus a sensible instruction set and encoding made by people who know what they are doing.