Earlier quoted context omitted.
I recently avoided a startup prospect because they were looking to build a car OS that wasn't hard real-time. The very idea that they're trying to develop such things that might intermittently pause to do some garbage collection is freaking terrifying.
What has hard real time have to do with garbage collection? You can have concurrent GCs (with [sub]millisecond pauses, or no Stop-the-world at all) but you also need 'hard real time' OS to begin with. Heck, even opening files is far from real-time. Then you need: not-blocking data strcutures, not just lock-free - that are much easier to develop. Pretty much you need forward guarantees on any data structure you'd use.
So I kind of would agree that producing any garbage during runtime makes it much harder to prove that a program can fulfill hard realtime guarantees.