isowner was implemented in version 5.61 for IRCX servers
23/09/99 - mIRC v5.61
Fixes/Additions:
2.Improved support for IRCX servers:
Added property $nick().owner, isowner if comparison, and "q"
property to $nick(#,N,aohvrq,aohvrq) where "q" is for owners.
Added on OWNER/DEOWNER events, work the same as on OP/DEOP,
and $mode().owner/deowner properties.
There is more in that same section that is somewhat related.
Click here for the full versions.txt file which is where that quote came from.
