There are many examples used as limb & joint protection for motorcycling and mountain biking. D3O, xmatter, SAS-TEC: brand-names of various non-newtonian shear-thickening substances used to allow flexibility and conforming to body motion but firming up under hard/sudden impacts to spread the forces. Not quite as soft or flexible as a fancy sci-fi material would need to be for a full-body armor, but certainly much more comfortable than the hard plastic materials previously used for protecting extremities.

These are often demonstrated by protecting a hand from a hammer blow, and I know from experience they are very adequate for protecting knees and elbows from trees, hard ground, and moderately pointy rocks. However, it's unclear if the shear-thickening is significant enough to protect against an actual sword strike or knife stab, and very unlikely that they could withstand a modern projectile weapon.

The idea is proven, but would need some further materials breakthrough to approach the level of sci-fi armor.

Not combat-armor, but a lot of motorcycle protective gear is exactly this. D3O is soft/flexible normally, but goes rigid on impact. I have plates of it in my own riding gear. I believe there's also some progressive versions that harden more or less based on the impact force.