They're stiff. Depending on how far back you're looking they either have a stiff leather sole with a leather "cleat" that fit into the pedal, or have a hard plastic sole like on current clipless shoes and then have a screw on plastic cleat that fits into the pedal. The cleats just had a slot in them that fit over the back edge of the pedal, so when you tightened down the strap it made sure your feet weren't moving anywhere.

You can still find classic slotted cleat shoes around sometimes, but they're not any better to walk in than current clipless road shoes, and much more difficult to get out of the pedal.