I think with the most recent chapters I have a better idea of what Master Rules are. After Ragnarok, a Rule/UMA is chosen to be a Master Rule and is now permanently added to the world, which is why Galaxy continue to exist from Loop 100 to Loop 101. When Apocalypse says that the rule will never disappear, he means even if you kill the UMA, the concept it represents will still exist. Which is why when Andy kills UMA Gravity and Magma last loop their concepts continue to exist and no major changes happen to the world. While when he killed UMA Insect and Thirst, you see the world change with those concepts disappearing. Apocalypse says the first 10 are the strongest. UMA Sick, the 10th Master Rule, has permanently exist since the 11th loop, which is why he remembers Juiz and why he was disappointed with being ignored by Fuuko this chapter. The Negators never had to experience the change of a world without sickness to a world with it. He has always been here to them.