Already in 4.0 (and earlier), the [n] format was only documented as being possible to use for lists - from the PATHS section of the confd_lib_cdb(3) man page there:
Thus instead of an actually instantiated key inside a pair of curly
braces {key}, we can use a temporary integer key inside a pair of
brackets [n].
It was indeed possible to use [0] for a container, unclear if it was a bug or a misfeature, but it was removed when fixing an error reporting issue for paths using [n] as documented - from ConfD-5.1 CHANGES (April 2014):
- CDB: If cdb_exists() was given a path with an instance index that was
out of range, it would fail with CONFD_ERR_BADPATH instead of returning
0. This has been fixed. Additionally, if instance index [0] was used for
a node that wasn't a list, it was silently ignored - now this fails with
CONFD_ERR_BADPATH.
There isn’t. Just drop it - saves 3 useless characters in the path…