In terms of dollar volume ARM is still the leader, especially for higher-end (application level MCUs) stuff. RISC-V MCUs with MMUs or MPUs are scarce at the moment.
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.
I do find it odd that you go on and compare to x86 marketshare however, the topic you've quoted is very clearly about MCU and whilst 8086 MCU still exists they haven't been used in greenfield projects for decades. Let alone any more recent x86 implementation.
Do you really think Chinese manufacturers are going to buy ARM MCUs when their budget for a controller is less than 10 cents?
ARM has long ceded this market to RISC-V. It's mostly focusing on high-end application MCUs and AI now.
And lots of newer stuff is making use of standardized boards like Raspberry Pi Pico (RISC-V and ARM hybrid) or ESP32 (RISC-V too on some versions).