Is there any alternative for SCRIPTABLE CART ? I have client script running with SCRIPTABLE CART enable which I would like to avoid. This is for SC and SCA version. Basically I would like to avoid call which goes through client script when scriptable cart is enabled. Can we do any customzation using extension on checkout page and turn SCRIPTABLE CART off?

Depends on what you're trying to accomplish and where. Do you have some details?

I have client script which is running when I add items into cart. And on every add to cart items client script 'Sublist' change event occurs which I would like to avoid. in that Sublist change event script we are calling some APIs which we don't want to do instead on that I would like to do that customizations using extension.

Does the data you need to retrieve have to come from an api call at the same time the line is added to the cart?

Yes

Are you editing something on the sales order item sublist?