Late 2004 is very far past the classic timeline, and why is an experienced player giving the suggestion of invising your pet at that time, if it had been out of the game for more than 4 years?
That thread even talks about pets being changed to beta neutral faction (won't get aggroed), with the ability to toggle that faction on/off, therefore making it unnecessary to invis the pet in PvE.
The ability to invis your pet was more likely removed after classic, maybe during the introduction of pet windows, or maybe because they were specifically trying to push the Pet Hold or Pet Suspend AA.
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