Quoted from HHaase:

Also, Williams (under the umbrella of Scientific Games) still actually OWNS all the intellectual property, and none of the other parties since then has actually purchased the IP, they just paid for the rights to use the licenses. Scientific still uses the old pinball IP from time to time as part of their slot machine lineup. (Black Knight, AFM, Funhouse).
I remember discussing this with Rick when I was first looking into selling my EPROM adapters, and he had to get their permission for me to migrate from multiple EPROM's to a single chip, even though the actual content was unchanged. IIRC it was the first time he handled a derivative work with them so it took a while to get sorted out.
-Hans