NOT ONE character in six books over twenty years mentions them even ONCE. Yet since their appearance in The Machine Crusade, there hasn't been a McDune book in which they aren't mentioned at least once. (MC: 10 mentions; BoC: 5; HuoD: 1; SoD: 1; PoD: 5; WoD: 1)
Not ONE reference in passing in all those books, even by "Oh, but listen to me prattle on, ha ha!" Leto II? Even the "Flying Dutchman of space" got one mention. Norma Cenva? She's in there, too. But the mysterious Muadru?
From their first appearance it was obvious from the form of the name that some connection was going to be made with "Muad'Dib", but it wasn't until PoD that it showed up:
"Your name comes from the Muadru?"
"There appears to be a linguistic connection between the Fremen and the Muadru, but the latter race vanished at independent sites all over the galaxy—suggesting a terrible cataclysm that took them all at once."
If "Chisra Sala Muad'Dib" in Winds demonstrated how little respect KJA or BH has for Arabs and their language (they didn't care enough to get the details right), the connection of "Muadru" and "Muad'Dib" demonstrated their ignorance. (In passing, there are problems with the science of a "terrible cataclysm" overtaking a space-faring race "all over the galaxy ... all at once" but the main problem is that Mr. "I write the science" is STUPID.)