Hello @ajaymeghtech
When the products get out of stock it will not proceed further because you will not able to Add that Product into the cart till then you have not enabled the Backorders, Then what is the use to stop this?
The below image is Quantity 0 (Out Of Stock) you will not able to the product into the cart
outoS.png
And now in the below image same product with Quantity 0 (Out Of Stock) and now you will able to Add to cart.
backorder.png
Please let us know if having further queries.
Thanks & Regards..!!