SOLVED: "You can only whisper to members of your alliance."

I found this addon that will mute this system message. I've tried chatter, prat, LFG board, get off my lawn, and a looot of other addons that I expected to filter chat message, but only this one specifically worked:

https://github.com/fondlez/MuteChatWrongFaction/

When you unzip the file you get a MuteChatWrongFaction-main folder. Erase -main from the folder name before loading the game.

The addon won't load if -main is in the folder name for some reason.