In his RISC-V Summit Keynote this morning, Meta's Prahlad Venkatapuram makes a compelling case for new RISC-V standard specifications for custom extensions, esp. "unify how these extensions are introduced to software". This has very close alignment with the objectives and use cases for the Soft CPU SIG's draft proposed Composable Extensions specification (which we are preparing as a new task group).
https://github.com/grayresearch/cx
https://github.com/riscv-admin/sig-soft-cpu/blob/main/TG/CX/CHARTER.md