It doesn't.
I just pulled the latest version at < and built it with `ASH_BASH_COMPAT` enabled. (It's enabled by default; see `shell/Config.in`).
$ bash -c 'a=(zero one two) ; echo ${a[2]}'
two
$ ./busybox ash -c 'a=(zero one two) ; echo ${a[2]}'
ash: syntax error: unexpected "("
$
The latest update was 2017-12-26 20:19.