Which soft sweater fabric stops a Sphynx from doing the stiff little purrito walk?

I've found that microfleece or anti-pill fleece works best because it provides enough grip and warmth without being too slippery. Your Sphynx will likely stop the stiff-legged walk once the fabric gives them traction and coziness, unlike satin or slick knits that cause that funny sliding motion.

Velvet or velour tends to give enough friction for their skin to grip, which calms that rigid leg shuffle. I've seen many Sphynx relax once they feel secure on a plush, non-slick surface.

Cashmere or a wool-cashmere blend gives just the right amount of cling without being too slick. My own Sphynx stopped that rigid leg shuffle as soon as I wrapped her in a soft cashmere sweater - the fibers grip their skin gently, letting them relax instead of fighting for traction.

A brushed cotton or bamboo-cotton blend with a slight nap does the trick. I noticed my own Sphynx stopped that awkward leg lift the moment I switched from a slick acrylic sweater to one made from organic cotton that had been brushed on the inside - the microfibers catch on their skin just enough to give them confidence in their footing, and the breathability keeps them from overheating.

A lightweight merino wool and silk blend-specifically a fine-gauge, slightly felted one-works wonders. I learned this after watching my own Sphynx, Mochi, do that hilarious, stiff-legged shuffle in a cotton sweater; the moment I put her in a soft, almost velvety merino-silk mix, she instantly relaxed, walked normally, and even started kneading with pure joy. The natural fibers create just enough micro-texture against their hairless skin to provide traction, while the silk prevents any static cling or overheating that makes them feel trapped.