OpenTitan _is_ a microcontroller, just one with a _lot_ of security hardware (and security proofs).
It's intended to be integrated into a larger SoC and used for things like secure boot, though you could certainly fab it with its own RAM and GPIO and use it standalone.
It's intended to be integrated into a larger SoC and used for things like secure boot, though you could certainly fab it with its own RAM and GPIO and use it standalone.