Answer by Hamfri for How to change border radius of bootstrap modal?

In Bootstrap 4 Try the following:

.modal-content {  -webkit-border-radius: 0px !important;  -moz-border-radius: 0px !important;  border-radius: 0px !important;  -webkit-border: 0px !important;  -moz-border: 0px !important;  border: 0px !important;}

border-radius removes the rounded corners while border removes all borders (left, right, top, bottom) around the modal.