[cpgxiii]

No it is not. No regulators has yet been prepared to certify a fixed-wing aircraft that is intrinsically unstable, even after decades of proven fly-by-wire development.

The 737 Max has different yoke force during pitch-up than predecessor 737 models, such that at higher angles of attack it does not natively require increasing yoke force to continue to pitch up. That doesn't mean it would pitch up uncontrollably. MCAS was designed to provide pitch-down force in these high-AoA cases so that yoke forces would be equivalent to 737 NG models and minimal training would be necessary to fly both.

[bruckie]

https://en.wikipedia.org/wiki/Relaxed_stability#Unstable_air... indicates that the MD-11 (which regulators certified) is aerodynamically unstable. (That plane has a compensation system, similar to the 737-MAX.) Is "intrinsically unstable" different from "relaxed stability" in some subtle way?

And while military planes are quite different from commercial planes, many (most at this point?) military jets are aerodynamically unstable.

[cpgxiii]

There are different kinds of "relaxed stability", largely depending on which axes of the aircraft are affected, and the magnitude of the instability.

Longitudinal stability is something of a special case, in that essentially all swept-wing aircraft are vulnerable to "Dutch roll" instability and are generally fitted with yaw dampers. Since such stabilizers are, practically-speaking, omnipresent, regulators are OK with using them in what is now a well-understood domain. While it can be unpleasant, all of these aircraft can be flown with a failed yaw damper - notably the 707 family has a particular proclivity for yaw instability, and while almost all civilian users opted for the yaw damper, the largest fleet user, the USAF, did not fit their KC-135s with dampers until well into their service life.

For good reason, pitch instability is a much more serious issue, and there has been very little interest in trying to bring to market a transport aircraft that required active pitch stabilization. Many, if not all, modern clean-sheet airliner designs are fly-by-wire due the the safety, performance, and efficiency improvements to be had, but they are all safely flyable in an "alternate law" (or equivalent) fallback mode.

Combat aircraft, generally speaking, aren't certified aircraft (they have no need be), for good reason - if you're flying a modern fighter and the FBW computers die on you, it's over, you eject. Understandably, that's not an option in a transport aircraft.