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.

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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.

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17/02/2006 - mIRC v6.17

Changes:
..
40. Fixed formatting of outgoing wallchops/users/voices.