I have a CRM solution using new unified client experience. I don't want to support legacy web client. When I create form of type "quick form create" inside our solution, the form itself is being shown also in legacy web client under the "+" button.
I figured out how to register custom script to say "sorry, please switch to unified client experience" but that is not very nice user experience.
Unfortunately that global Navigation bar is not customizable in a supported way. The workaround what you have is supported choice.
You can try unsupported DOM manipulation to hide that element similar to this discussion.