Yes and no. You could argue that use of passwords or private keys is security through obscurity, but that definition is pretty meaningless. Security through obscurity doesn't really apply when you can make the search space arbitrarily (often exponentially) big very cheaply, as is the case of URLs here.
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Unless they know what counter you're using or you make it public, it's unreasonable that anyone could mess with your counter.
...or you choose convenient, meaningful $group and $id names, which makes even unintentional collisions not at all unlikely.
This would be an example of security through obscurity.
Yes and no. You could argue that use of passwords or private keys is security through obscurity, but that definition is pretty meaningless. Security through obscurity doesn't really apply when you can make the search space arbitrarily (often exponentially) big very cheaply, as is the case of URLs here.