I have serious doubts whether this ever happened. Certainly, the rule always was that if a city had either reached chapter 5 (I think) or had purchased diamonds at any point, then that city would remain for ever more. I haven't seen any evidence of this changing. As
@Hekata points out, I have inactive cities in my neighbourhood that have been inactive since the time when I was moved there (the one and only time I was ever moved). This was in "the great move" when many people got moved around the map all in the space of a day or two. I think that was August 2017. As I've mentioned before, I have my eye on one particular neighbour. At almost this exact time in 2017 he deleted his whole city, bar the things that can't be deleted, and he left. I believe his city is chapter 5. That same desolate city still sits there to this day.
What did happen was they increased the frequency after which the tiny cities are swept off the map. That much is evident.
As far as I know if a city is removed then it is also removed from any fellowship they are in. To all intents the city ceases to exist. The problem is many fellowships keep larger inactive fellows in their group because they believe it somehow gives them extra status. So it stands to reason that these are the same inactive cities that are never being removed.
We could go on having this conversation, and it's probably pointless now, but I still don't understand why those cities can't be moved out to the edge of the map, even if Inno doesn't want to remove them completely, though I do realise that it is not a completely quick and straightforward process to move cities around.