Is scoliosis spinal surgery bad for your flexibility?

Yes: Scoliosis surgery is performed as a fusion. This result in ablation of motion of the involved segments. The more segments involved the more restriction of back motion.

Fused not flexible: The reason for scoliosis surgery is to correct the deformity and/or prevent any worsening. To do this, a fusion is done which essentially stops any motion through those fused areas. However, flexibility in terms of bending can be maintained by keeping your leg and hip muscles flexible