I remember, when I first joined the Forum - about three years ago, coming up? - that one of the most lively topics, even back then, was the subject of City moves... how and why and when they happened, and especially the Big Question: does spending Diamonds in-game mean that your City is never moved, even if long abandoned? And this topic is still just as interesting and, it seems, just as much debated even now! Very interesting.
My experience, as is true of all of us, of course, is merely anecdotal, but in my own Neighbourhood on EN1 Arendyll, which is reasonably centrally placed and also quite active (between 10% and 20% live Cities at any given time, including start-ups and mid-high Chapter players), I find that around 10% of the dead Cities are removed reasonably often - every 2-6 weeks or so - and replaced either with Gold Mines (for anything up to a few weeks), or with other Cities, of which some are entirely new start-ups and others are relocated Cities in later Chapters, although none, so far, has been above Ch.V.
Most of the relocated Cities are, it seems, already dead before being moved, which would seem to indicate either that no 'storage' of dead Cities is taking place, or at least that dead Cities may be moved multiple times before being 'stored'... which would seem to be a waste of effort, but perhaps there is actually a good reason for this? Do others also see already-dead Cities being moved into their Neighbourhoods, too? I'd be interested to know.
This same degree of moving/removing/relocating Cities around me has been going on ever since I started my Arendyll City, some 15 months ago, so I personally have never noticed any break in the removal/relocation schedule which might suggest that the system is in some way intermittent. Something which has caught my attention however, of late (since maybe three months ago?), is that almost all of the Cities, whether tiny or more advanced, which appear near me as the result of having been moved into an empty space will have a four-digit code [precisely four, no more nor less] appended to the City Name (for example, my own City would be renamed to 'Laurelin-3408', or similar). So far, any 'Code Cities' which are moved in are moved out simultaneously, although new (and dead) 'Code Cities' may or may not replace them. I haven't seen any City which I know was once live but is now dead actually acquiring these four-digit codes, even though some of them have been removed along with the 'Code Cities', at times, so I can only assume that the code is added at some point after the removal of such a City, but before its replacement elsewhere (and/or, for all I can tell, in 'storage' of some kind?).
This coding is so consistent in style (and so unlikely to be the names chosen by players themselves) that I can only assume it has something to do with how Cities are identified by the City moving system. It would also suggest, perhaps, that no more than 9999 Cities can be moved simultaneously?
Of course, this new development in the Saga of the Moving Cities doesn't really help any of us to understand precisely how it all works, but still, maybe any new info is better than none...?