The English Constitution would seem to be one of the ur-examples of this. Insofar as the thing does legally exist, but what it represents is an abstracted conception of common law with no coherent identity or definitive “completeness” in itself.
Feels like I’m not getting the question across clearly. For example “DC” is a true metonymy for “US federal government” which is a fuzzy but real thing.
“Area 52” might be a good example… metonymy for a vast alien research military industrial complex that doesn’t actually exist