Clicking through links eventually led to https://lowrisc.org/ibex/ -

> Ibex® is a small and highly configurable open-source RISC-V embedded processor available under an Apache 2.0 licence. It is formally verified and very well validated, and it has excellent toolchain integration, which has led many companies to use it in their commercial SoCs.

> [...]

> Ibex is the main CPU in the OpenTitan® root of trust, which has brought the quality of the design and documentation to new heights.

So that's neat.