> RISC-V MCUs with MMUs or MPUs

MPUs are more common. The Physical Memory Protection option is basic and allows ranges of memory to be set unavailable in user mode. A few well-defined ranges do let you lock a user process down securely but it's not a real MMU. Low-end microcontrollers don't have enough RAM to warrant a real MMU.

The RISC-V core in the Raspberry Pi RP2350 has PMP as do the ESP32 cores.