At the risk of seeming like the guy who watches random tv shows and finds internal reserves everywhere, I watch random tv shows, and I find internal reserves everywhere. This is an exception because (1) it's not Finnish! And (2) I thought I found some, but I was wrong, but then I found some.
Right from the very first scene of Mare of Easttown (in the location I think seen above) I thought I was looking at houses which backed on to internal reserve spaces of some sort. Clearly back doors of houses within a block were visible to each other, and somehow between those back doors there was publicly accessible space. However, it wasn't clear - beyond low-rise fences - what was between those back doors. That was, until I went looking at a very convenient website that details (apparently) all the locations of the show. From this I came to appreciate that the place that piqued my interest (above) is actually in Marcus Hook, and constitutes a narrow grid block which has a public pathway down the middle, and very long backyards, but no shared space (aside from the path). You can see it (or at least an example of it) here.
Oddly though before I found the abovementioned website, I went searching in both Aston Township and Chester County. Aston Township features many internal reserve spaces. They include Hidden Park, Aston and Bridgewater Farms Park and Sampson Park also in Aston Township. These don't seem to feature in the show at all (well, I haven't finished watching it yet)...
Update: after watching the entire show, and without wishing to spoil anything, I will say the rear path plays a key role in the denouement of the show. It's very clear that the gardens/yards at the rear of the Marcus Hook blocks are big, empty and essentially useless spaces.