"Mission Critical" - seven 9s reliability as being a minimum
"Secure" - B2 or EAL5 as a minimum, can't have known exploits, security kernel must be amenable to being provably correct (whether or not it has actually been proven)
"Real-Time" - I'll accept "fixed number of CPU cycles in a given window, doesn't matter when in that window", -or- "fixed start and end times in which the program will be running, doesn't matter exactly how many CPU cycles were used", soft real-time is not really "Real-Time".
And so on. Real-Time has multiple definitions, so I simply ORed the two usual ones together and excluded the one that should never have existed.
Some of these are orthogonal, some have trade-offs and some are mutually exclusive. Which is what has changed, as new techniques and methods are discovered. "Mission-Critical" is orthogonal to "Real-Time" up to the point where sufficient validation of pre-conditions permits deadline guarantees to be met.
Where it is NOT possible to validate pre-conditions to the extent needed to ensure the probability of an error resulting in a failure condition is below the threshold guaranteed (99.99999% reliability = 0.00001% probability of a catastrophic error) THEN you cannot have something that meets both requirements, you have to trade off.
However, for most practical purposes (and I include NASA's rocketry, Boeing's military aircraft, etc, to be a practical purpose), well-written software, developed using an established and recognized software engineering technique, properly tested, is very unlikely to require validation that is so extensive that real-time guarantees can't be met.