It is a very interesting question. If an aircraft is certified single pilot by the respected authority then that’s what it is as per the manuals etc.
Now things change with different authorities when it comes to private flying or commercial flying. For commercial ops some require a second pilot or ‘safety pilot’ in the RHS that doesn’t even need to be type rated. At other times during private flying it can be an insurance requirement to keep premiums down. Pretty sure actual requirements are down to the authorities.
Some of the light jets can be a handful, they are fine while everything is going fine, however throw in at system failure, wx, etc all while belting along at 400kts they can be a handful, hence why some authorities required two pilot commercial ops.
Well that’s my take on it through personal experience.
