Is foot fetishism (foot worshipping) considered a form of shirk with Allah—we seek refuge in Him—?

What you have mentioned is a deviation and an illness that requires treatment. The heart's inclination towards this deviation is neither shirk (polytheism), nor kufr (disbelief), nor a sin, as long as it does not lead to committing something forbidden. Naming it "idolatry of feet" does not entail any legal religious consequence.