There's a toenails section coming to CAS and so I can only assume we get Pedicures as well in Spa Day. Also, there are a variety of lengths of fingernails in the pictures - Spa Day provides the extra fancy long ones and Manicures which apparently have the gameplay of gradual decay. But there are a few shapes in Base Game too.
However it's a change to CAS - not the biggest in the world, but definitely something that will affect your Accessories Mods. No need for any fingernail mods any more!
The nice thing about nails in Base Game is that we may get more nails in the future. Different textures in pack refreshes, or hideous claws for our occult creatures. It's like when we got Teeth. You don't notice unless you zoom in tight, but hey, that's what the Photography career is for! And hey, fancy fingernails works well with Get Famous.