It might be an api command that wasn't added to the Classic system.
It was originally added in 3.0 so would not have been available in Classic or BC. So, unless it was one of those quality of life features they kept, it may have been removed from the system for those two systems.
https://wowpedia.fandom.com/wiki/API...eatStatusColor
By the looks of this github project comment .. it appears that was the case indeed.
https://github.com/EsreverWoW/ClassicThreatMeter