Do I have to pay for battery subscription after purchasing at INR 149000 for 450 Plus or INR 159000 for 450X?

Of course not. At these prices, you would have bought the scooter with the battery. On top of this, you can opt for the connected services pack at ₹300 per month or the Ather Charge pack (incl. unlimited fast charging in Ather Grid, connected services) at ₹400 per month.