There are 4 repositories under user-namespaces topic.
Low-level unprivileged sandboxing tool used by Flatpak and similar projects
Simple desktop application sandboxing tool for GNU\Linux
Very experimental docker authorization plugin, disabling some trivial ways of gaining root via docker
Experiments with unshare
Kernel patches for non-init user namespace on FUSE filesystem
Nesting containers with podman
A nix shell running in a (thin) container