Because it has been deprecated since v5.81.
Code:
Versions.txt: 09/11/2000 - mIRC v5.81
 
12.Added $sdir(dir,title) and $sfile(dir,title,oktext), title and
   oktext are optional.
 
   The $sdir, $file, $hfile, and $dir identifiers are still supported
   but are no longer documented in the help file.