I was partly incorrect, I just found these in
versions.txt. It seems mIRC has a built-in workaround for servers that don't support the wallchops command, for backward compatibility
it just calls the onotice command, which has its own routines for dealing with non-support.
28/08/97 - mIRC v5.1
Changes:
..
114. /wallchops command is now translated into the /onotice command.
-
26/04/2001 - mIRC v5.9
Changes:
..
44. Added support for numeric 005 WALLCHOPS token, mIRC will use
/NOTICE @<channel> :<text> if this token is set, instead of
using multiple targets.
-
17/02/2006 - mIRC v6.17
Changes:
..
40. Fixed formatting of outgoing wallchops/users/voices.