Prior to Starship, just getting large enough structures and mass into space has been the limiting factor. There is a minimum diameter for spin gravity to avoid disorienting vestibular side-effects. Spinning two spacecraft attached by a tether might work for short-term use cases, but I don't know how feasible it is to keep that stable for months or years. Transporting adequate volumes of water shielding is probably infeasible for SpaceX's planned Mars mission profile:
- you need 1 meter of water shielding to reduce radiation to safe levels[0]
- each cubic meter of water is 1 metric ton
- Starship's circumference is ~28 meters
Assuming you only shield 4 meters of the length of the ship, not counting the top and bottom cross sections of the cylinder, you've already blown past the 100T payload capacity of Starship.