I was long ago convinced by the compelling arguments of Doris Srinivasan that the Indus seals having nothing to do with either Śiva or yoga.
Surely this issue isn't "much contested" any more, at least scholarly circles?
Further discussion in Samuel, Geoffrey. The Origins of Yoga and Tantra: Indic Religions to the Thirteenth Century. Cambridge: Cambridge University Press, 2008, in Asko's work and elsewhere.
Best,
Dominik