Christian Brauner [off-list ref]:

2. Incomplete - misses inactive namespaces that aren't attached to any
   running process but are kept alive by file descriptors, bind mounts,
   or parent namespace references

I don't like word "inactive" here. As well as I understand, it is used in
a sense, which is different from later:

(3) Visibility:
    - Only "active" namespaces are listed

-- 
Askar Safin