I believe kernel 5.19 added ID-mapped-mount support for overlayfs (need to double-check). If true, then we will adjust sysbox to use ID-mapped-mounts for the container’s rootfs too, and at that point ID-mapped-mounts would essentially replace shiftfs for all practical purposes.
However I have not been able to find any discussion of how or when this is being implemented.
assuming that in fact overlayfs supports ID-mapped-mounts, this will be included in the ~v0.7 release of Sysbox. Not sure on the timeline yet, likely ~Feb 2022.
In any case, overlayfs support for ID-mapped-mounts is a “nice-to-have”, but not a “must-have” as mentioned in my comment above.
Should be present in the upcoming Ubuntu 23.04 release (Lunar Lobster), due April 2023.
NOTE: the upcoming release of Sysbox (v0.6.0) will automatically check if shiftfs works on the host or not, and adjust accordingly. In platforms where it works, it will use it as needed. In platforms where it does not work, it will use an alternative mechanism. The new Sysbox release will also automatically check if the kernel supports ID-mapped mounts (kernel 5.12+) and overlayfs on ID-mapped mounted lower dirs (kernel 5.19+), and use both of these features. The latter one really makes shiftfs unnecessary going forward.