Hi Stefan,

SDF modelers can basically use any shape the developer adds to the engine. If you look at Clayxels and Mudbun...

https://andrea-intg.itch.io/clayxels

http://longbunnylabs.com/mudbun/

... those have splines, and in Cinema 4D's SDF implementation you can also use splines and any other shape that's available.

Regards!