I think rather than disable city growth, they should start fudging the population numbers at least of those rendered and ‘doing stuff’ … So for example one civ becomes 10 or to that effect in order to remain in an optimised state of x max active civs visibly wandering..
I mean, its all good and well having 10,000 civs wandering around a map, but you can only watch a handful and since it is contributing to killing the performance leading to the situation of not being able to continue to play the same save past a certain point, it makes sense that it needs changing… that is of course assuming this is where the issue is.
There’s other things killing performance too though, most UI windows take a big chunk for a single window, not to mention multiple, such as ‘old vehicle warnings’ or buying x vehicles to replace a fleet.
Hopefully… they can fix / optimise it at some point. The last patch made things marginally better, but there is a lot more needed to keep the game playable.