Is this more about the ABI? I couldn't find incompatible userspace ISA changes since many years (things sounded to have settled down since a while even in 2016: https://riscv.org/wp-content/uploads/2016/06/riscv-spec-v2.1...). The RISC-V Linux port was also merged to mainline in 2017 and presumably been living under the stern binary & ABI compatibility rules there.
Since ISA collections affect ABI in RISC-V, getting an agreement of the baseline for RISC-V ISA for Linux-based systems was extremely difficult. On top of that, the ABI stabilization work couldn't begin until the required ISA extensions for Linux were locked in. So it was all a mess.
