Are weapon grips, such as sword handles (not including hilt guards and pommels) required to be covered in foam?
No. The majority are, with a leather grip over that, but there are wooden, bone & antler grips on weapons out there. As you surmise, pommels and guards need to be foam.
It’s more common on daggers - due to being more props/ornamentation that actual weapons. I carry an antler-handled knife.