E.g. Snap sandboxing: https://snapcraft.io/docs/security-sandboxing
It doesn't help a lot that instead of having a explicit "can I have a host file" API, the apps are supposed to use a framework that invokes "portals".
The ability to let any ring (such as a user ring) handle the library and system calls of an outer ring (such as a container ring) seems like it would be convenient.
Alternately, some microkernels have a more peer-to-peer setup which could work as well.
Sadly both most used userspace distros and the kernel they come with are borderline dangerous and shouldn't be any trusted by people using them, even if they're technical enough to feel comfortable within their environment.