I've been told that, although all 737 Max aircraft are equipped with MCAS, the system can be deactivated by the individual airline. I've also heard that US airlines never activated MCAS in their 737 Max aircraft. Any truth to either?
No, MCAS was required for the 737 MAX to be certified by the FAA. It cannot be deactivated by any airline, including airlines in the U.S.
"M.C.A.S. was necessary then for the airplane [737 MAX] to be certified by the F.A.A. to have met all of the regulatory design requirements for stability and control."