He did do it, it's not a myth. He says so himself in various documentaries I've seen. But to that, I say (as I've stated before), So what? It's just spit! Much worse has been done to fan by musician/idol. Perspective is needed on this. He already apologized to the spittee (heh...just made that up) in the form of The Wall.
I wonder whatever became of The Spittee? It's not ever like he came out on some bad TV talk show (like the Rock Star/Fan abuse Oprah series) years later saying how traumatized he was by being spat upon. Probably was too drunk to notice at the time...probably was gobbed on unintentionally by his drunken friends too. Probably was just there to hear "Money."