Sorry for the miss.
root@server:~# lsb_release -a
No LSB modules are available.
Distributor ID: Debian
Description: Debian GNU/Linux 10 (buster)
Release: 10
Codename: buster
root@server:~# lxc config device set c0 disks shift=true
Error: Failed to start device "disks": Required idmapping abilities not available
root@server:~# lxc config device set c1 disks shift=true
Error: Failed to start device "disks": Required idmapping abilities not available
Seems not good, however
root@server:~# modinfo shiftfs
filename: /lib/modules/5.4.114-1-pve/kernel/fs/shiftfs.ko
license: GPL v2
description: id shifting filesystem
author: Christian Brauner <christian.brauner@ubuntu.com>
author: Seth Forshee <seth.forshee@canonical.com>
author: James Bottomley
alias: fs-shiftfs
srcversion: 225AF9C817280FFD72CB9A8
depends:
retpoline: Y
intree: Y
name: shiftfs
vermagic: 5.4.114-1-pve SMP mod_unload modversions