Earlier quoted context omitted.
The technical mechanism is exactly the issue to figure out, there's a reason why projects don't have this and it's because different implementations have different tradeoffs.
No, the reason people haven't been doing this -- and we've had technologies for ages -- is that it's a huge pain in the ass, a "tax", that it's hard to get developers inside a company to pay, much less participants in open source ecosystems. Look at how snap, flatpak, etc. provoke people to just turn off security rather than deal with breakages. A new language won't help because the problem is social, not technical.…
Sandboxing almost always involves having a dedicated, privileged service, due to how operating systems have designed things. It requires platform specific code.
This is a technical problem and a social problem, there's zero reason to believe it's just one.