Since Linux 5.8, proc can be mounted with options subset=pids to hide all non-task-related information. This is missing from proc(5). Original patch: https://lore.kernel.org/all/20200423200316.164518-6-gladkov.alexey@gmail.com/ The kernel documentation shows the option: https://www.kernel.org/doc/html/latest/filesystems/proc.html#mount-options This option is useful when mounting proc for a container nested for example in a Docker container that hides certain parts of /proc, otherwise one is not allowed to mount /proc (https://lkml.kernel.org/lkml/cover.1597931457.git.gladkov.alexey@gmail.com/). It might be useful to add a note about this as well.
Actually, skip my last paragraph, it seems this patch did not get merged yet. Last iteration I found is at https://lore.kernel.org/all/cover.1626432185.git.legion@kernel.org/. But the mount option does exist.