iOS 9 UITableView separators insets (significant left margin)

If you want to do it in interface builder. The default separator inset is Automatic. Change it to custom by selecting the dropdown.

Okay, I have found out the solution. The only thing required for that is to set on the presenting instance of UITableView that flag cellLayoutMarginsFollowReadableWidth

myTableView.cellLayoutMarginsFollowReadableWidth = NO;

I wanted to find some reference in the documentation but it looks like it is not ready yet, only mentioned on diff page.

As the flag was introduced in iOS 9 for the backward compatibility you should add a check before trying to set it:

if([myTableView respondsToSelector:@selector(setCellLayoutMarginsFollowReadableWidth:)])
{
    myTableView.cellLayoutMarginsFollowReadableWidth = NO;
}

For Swift 2.0 you can use #available to check iOS version.

if #available(iOS 9, *) {
    myTableView.cellLayoutMarginsFollowReadableWidth = false
}

Moreover you need to compile it with Xcode 7 or above.

EDIT

Please keep in mind that this is the only required fix if your separators looked "fine" up to iOS 8, otherwise you need to change a bit more. You can find info how to do this already on SO.