Rootless OverlayFS is supported since kernel 5.11.

Older kernel releases didn’t support rootless OverlayFS, though Ubuntu supports it by patching the kernel.

Debian supports rootless OverlayFS too, when overlay.ko is loaded with a custom modprobe option permit_mounts_in_userns=1. However, Debian version of rootless OverlayFS (before kernel 5.11) is known to be broken as of April 2021, while Ubuntu version seems stable.

On other distros, Rootless Containers typically use fuse-overlayfs instead of the real OverlayFS.