what platforms don't have `dirname`?

The autoconf info pages say

  ‘dirname’
      Not all hosts have a working ‘dirname’, and you should instead
      use ‘AS_DIRNAME’ (*note Programming in M4sh::).  [...]

Looking into the git repository log it seems this remark is oooold,
essentially written before 1996.  I now wonder whether this has any
relevance today.  Are there still platforms supported by autoconf that
don't have `dirname`?

My question arises from the fact that I often look up the autoconf
info pages to check whether certain common tools are portable or not
in shell scripts.  However, in this particular case it seems to me
that the advice, while useful for autoconf, is very outdated.


    Werner