If you setup user namespace, the child processes will inherit that namespace. The difference is that plan9 is fully built on this idea and isn't multi-user, on linux you have to opt-in to this. It's very useful and underused (mostly used by containers). I wanted to ship my AWS lambdas this way, but sadly AWS lambdas don't allow user namespaces.
Plan 9 is very multi-user, namespaces are actually one component in how it is multi user specifically. A given terminal may be designed to only have one user on it but CPU servers are still multi-user multiplexers, that part has not been given up.
Yes, by multi-user i meant, plan9 doesn't have the unix/linux user model, but rather "multi-user" is done with the namespaces. Container people would be more familiar with the plan9 way.
https://github.com/aws/aws-lambda-base-images/issues/143