Could all mammalian receptors be described as allosteric?
"Allosteric" refers to an alternate site of action, but it is not limited to a discussion of enzymes. For a protein that is a receptor, allosteric modulation refers to modulation of that receptor away from the binding site or "active site" of the endogenous ligand.

Although it is true that the receptor may have enzymatic activity when a ligand is bound, it would be non-standard to use the term "allosteric" to refer to the normal ligand binding site, even if this terminology is technically correct with respect to the enzymatic activity.

There are many receptors which are not enzymes at all: a great example would be ligand-gated ion channels - all of the activity of these proteins is due to their conformational changes, and not due to any catalyzed reaction.