In Konsole settings -> "Tab Bar / Splitters" there's an option called: Show 'New Tab' button in the Miscellaneous category. It is disabled by default, and I think it should be removed for two reasons: 1. Confusion. Maybe someone could mistake it for the "New Tab" button that is right at the side of "New Window" at the top-left of Konsole. 2. Duplicate. There's already a "New Tab" button right above it, so this new "New Tab" button is unnecessary. Thanks.
I'll defer a decision to the Konsole maintainers, but for what it's worth - those points don't apply to users who choose to not show the toolbar in Konsole :-)