This is only partially true - the only cars which used "real" drive-by-wire (wheel -> computer -> steering actuator) were a few Infiniti vehicles, and even then they had a clutch which could re-engage a physical steering column.
Rather, a few cars in the early 2000s and many since the early 2010s or so offer electric power steering assist: the steering wheel is still very much connected by a physical steering column to the steering rack, and normal steering input is purely physical - there's just also an electric motor attached to the rack to provide the usual power steering boost. And, that power steering assist can be controlled over a non-diagnostic CAN bus to facilitate LKA.
Rather, a few cars in the early 2000s and many since the early 2010s or so offer electric power steering assist: the steering wheel is still very much connected by a physical steering column to the steering rack, and normal steering input is purely physical - there's just also an electric motor attached to the rack to provide the usual power steering boost. And, that power steering assist can be controlled over a non-diagnostic CAN bus to facilitate LKA.