Economics is not a STEM subject, nor are any other social sciences.
As above, they are not, as they have their own methodologies that set them apart from the (natural) sciences subjects so are not STEM subjects. Notably they are not categorised as STEM subjects by SFE in which courses they determine eligible for second degree funding (which among other areas is available for part-time STEM courses).
The one subject which is a bit of a grey area is psychology as unlike other social sciences which may use aspects of the applications of science (e.g. archaeology or biological anthropology) psychology is somewhat unique in utilising the scientific method in experimental work extensively as a core aspect of the field (as opposed to something which is sometimes used in particular areas like economics or sociology where sometimes experimental approaches are used).